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Start with the network: test latency between the two computers, and use Ethernet if Wi‑Fi is responsible. If latency is normal, check logging level, recent updates, clipboard size and transfer behavior, macOS Handoff, antivirus permissions, and screen-layout settings. The exact labels differ by product and operating system, so treat the Synergy steps below as product-specific guidance rather than universal software-KVM rules.
What a software KVM can—and cannot—do
A software KVM lets one keyboard and mouse control multiple computers through a local network. Depending on the application, it can also synchronize the clipboard. Deskflow describes this arrangement as keyboard, mouse and clipboard sharing over TCP, explicitly “without video”: each computer continues to render its own display. If you expected one computer’s monitor output to travel through the KVM, that is a hardware-KVM function, not a normal software-KVM feature. See the Deskflow project description for that distinction.
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Because input and clipboard data travel over the network, delay can come from Wi‑Fi, routing, filtering software, an overloaded transfer, or the KVM application itself. A cursor that feels slow, a switch that never occurs, and a clipboard that stops synchronizing are related symptoms, but they do not have the same fix. Use the branch that matches what you see.
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1. Test the local network before changing application settings
Intermittent lag on two Wi‑Fi computers is most often a connectivity problem. Synergy’s support guide states: “The most common cause for a mouse cursor to lag is a bad Wi-Fi connection.” Run a ping test from one computer to the other, following the method in Synergy’s cursor-lag guide. The result is useful for spotting packet loss or an unstable connection; the documentation does not define a universal acceptable-ping number, so do not treat an arbitrary threshold as a diagnosis.
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- Test while the lag is occurring, not only after it disappears.
- Repeat in both directions if your operating system and tools allow it.
- Note whether the delay is continuous or appears when the pointer crosses between computers.
If the ping test points to Wi‑Fi, connect the computers with an Ethernet cable. This is Synergy’s recommended remedy when wireless networking is the cause. A computer without a built-in Ethernet port may need a USB Ethernet adapter. Use the adapter only as a way to establish a wired network path; it does not improve a problem that is actually caused by logging, clipboard transfers, or permissions.
2. Reduce excessive logging
When the computers are already wired, or the ping test looks healthy, check the KVM’s logging setting. In Synergy, set logging to Info on every computer. The vendor warns that higher debug or verbose levels can themselves contribute to lag. Reproduce the problem after changing the level so you can tell whether the setting made a difference.
Do not delete logs before recording the time and symptom. If the issue continues, those entries can help support or an administrator correlate a switch, disconnect, or clipboard transfer with the delay.
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If lag began immediately after an application update, first record the current screen layout and other settings. Then follow the installed product’s documented reset or reinstall procedure. Synergy’s basic troubleshooting guidance includes a clean reset/reinstall for problems that start after an update. A reset can erase layouts, server addresses, permissions, and other configuration, so export or copy settings when the product supports it. On a managed computer, ask IT before resetting or reinstalling; security policies may restore the same problem or prevent the application from running.
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Keep the product family and version the same on all participating computers while troubleshooting. Synergy 1 and Synergy 3 have different behavior and settings, and a configuration made for one should not be assumed to apply to the other.
4. Check whether a large clipboard transfer is holding up input
A cursor that freezes just after copying an image or a large block of data points to clipboard transfer rather than ordinary pointer latency. Synergy’s copy/paste documentation for version 3.7.2 says the default copy limit is 4 MB and can be changed to a whole-number value from 1 to 1000 MB. Increasing that limit can make lag worse because more data is allowed to move through the same connection.
For a reproducible test:
- Copy plain text and move the pointer across computers.
- Copy a small image and repeat the switch.
- Try the same action with a large image or file-like clipboard payload.
- If only the large payload causes the pause, lower the clipboard size limit or disable clipboard sharing temporarily to confirm the cause.
Do not present the values above as limits for every software KVM. They are Synergy 3.7.2 settings. Synergy’s documentation also describes changed transfer behavior in Synergy 3 v3.8.0-beta and Synergy 1 v1.22.0-beta and later: clipboard data is sent after mouse movement rests and in pieces so input is less likely to wait behind the transfer. Those are beta/version-qualified behaviors, not a guaranteed fix for every stable release or derivative project.
5. Test macOS Handoff when a Mac pauses during clipboard use
Synergy documents an interaction with macOS Handoff in which cursor movement can pause. On the affected Mac, temporarily turn Handoff off and repeat the exact action that causes the freeze. If the symptom disappears, restore Handoff and disable Synergy clipboard sharing when you do not need cross-computer clipboard access. If turning Handoff off changes nothing, restore it and continue with the network, logging, and security checks instead of leaving a feature disabled unnecessarily.
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6. Check antivirus and endpoint controls without weakening security broadly
Antivirus or endpoint-security software can inspect or block the KVM’s network traffic. Check whether the application has permission on the trusted local network and whether a recent security-policy change coincides with the lag. Use a short, supervised test according to your organization’s policy; do not permanently disable protection. Managed devices should be tested with IT, which can create a narrow application or network exception and then verify that protection remains enabled.
When the computers will not connect or switching fails
Use the same product family and a supported version
Install the same KVM product family on every participating computer. Synergy’s basic troubleshooting article distinguishes Synergy 1 and Synergy 3; mixing them can leave the computers unable to establish the expected connection. Check the current version supported by the product you selected and update all peers consistently before changing more settings. The guide lists Synergy 3.7.2 and Synergy 1.21.4 in its current troubleshooting context, while the lag article discusses additional beta behavior, so preserve those distinctions when comparing what you see.
Verify the network and the screen layout
- Confirm that both computers are on the same local network or otherwise have a permitted route between them.
- Inspect the KVM layout so the screen edges that should connect are adjacent. If the remote computer is placed above the local display in the layout, moving through a left edge will not switch to it.
- Check that the server/host address and the client computer name match the product’s configuration exactly.
A valid layout does not prove that traffic can pass. It only determines where the pointer is expected to cross after the connection succeeds.
Check VPN, firewall, and antivirus filtering
A VPN can route traffic away from the local interface or apply rules that block peer-to-peer connections. Disconnecting from a VPN for a brief, authorized test can show whether routing is involved; reconnect immediately if policy requires it. In the firewall or antivirus control panel, allow the KVM application on the trusted network rather than opening unrelated ports or permitting public-network access. Synergy’s basic troubleshooting steps specifically call out VPN, firewall, and security settings as common connection checks.
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Release a locked cursor
If the application appears connected but the pointer will not leave one computer, check cursor-lock settings and the Scroll Lock key. A lock mode can intentionally keep input on the current machine, making a working connection look broken. Toggle the configured lock control once, then test a screen-edge crossing again.
Allow for desktop-specific Linux behavior
Do not assume that a Linux desktop using Wayland will behave like an X11 desktop. Synergy’s general checklist advises checking what works on the particular desktop environment, and support can differ by desktop, product, and version. Record the distribution, desktop environment, display protocol, and KVM version when seeking help. There is no universal Wayland clipboard or pointer fix established here.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Fix clipboard sharing when pointer control still works
Confirm that clipboard sharing is enabled in the installed product
Clipboard synchronization is a feature setting, not a requirement for keyboard and mouse sharing. In Synergy 3, copy/paste is enabled by default and configurable; open the copy/paste settings for the installed version and verify that sharing has not been disabled. Other products may use different names or defaults, so use their documentation rather than copying Synergy’s labels.
Separate a transfer-size problem from an operating-system conflict
Test with a short text string first. If text works but images do not, inspect the product’s clipboard size limit and transfer behavior. If nothing transfers in either direction, check whether the operating system or another clipboard-sync utility is taking ownership of the clipboard. Disable the competing utility only for a controlled test and restore it if it is not involved.
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On macOS, use the Handoff test described earlier. If Handoff is the trigger and cross-computer clipboard is unnecessary, leaving KVM clipboard sharing off is a targeted workaround while retaining Handoff. If clipboard sharing is essential, keep Handoff enabled only if testing shows it is not responsible and investigate product-specific updates or support guidance.
Interpret clipboard results on Linux cautiously
Desktop environment and display protocol can affect clipboard access. The available guidance does not establish one fix that works across Wayland, X11, and every Linux desktop. A useful report includes whether keyboard and pointer control work, which direction clipboard data fails, the data type (text or image), and the exact OS, desktop, protocol, and KVM version.
A symptom-to-action checklist
| Symptom | First check | Next action | Important qualification |
|---|---|---|---|
| Cursor is intermittently slow on two Wi‑Fi computers | Ping between the computers while the lag occurs | Use Ethernet if Wi‑Fi is implicated; a USB Ethernet adapter may be needed without a port | No universal ping cutoff is established |
| Lag persists on Ethernet or with normal ping | Logging level | Set Synergy logging to Info on every computer | Higher debug/verbose levels can add overhead |
| Lag started after an update | Version and saved layout | Follow the product’s reset/reinstall guidance, preferably with IT on managed devices | Reset/reinstall can erase settings |
| Pointer freezes after copying a large image | Clipboard size and transfer behavior | Lower the limit or temporarily disable clipboard sharing | Synergy 3.7.2 documents a 4 MB default and 1–1000 MB range |
| Mac pauses during clipboard use | Handoff | Test with Handoff off; restore it and disable KVM clipboard sharing if that resolves the issue and sharing is not needed | Documented as a Synergy/macOS interaction |
| Computers never connect | Product family, network, layout | Check VPN, firewall, antivirus, host/client settings, and screen-edge adjacency | Security changes should be narrow and authorized |
| Connection works but pointer will not switch | Cursor lock or Scroll Lock | Release the lock and test the configured edge | Lock behavior is intentional on some setups |
| Pointer works but clipboard does not | Clipboard setting and competing sync tools | Test plain text, then images; check OS and desktop-specific behavior | Wayland support varies by product and desktop |
How to collect useful evidence before escalating
- Write down each computer’s operating system, desktop environment where relevant, KVM product family, and exact version.
- Describe whether the symptom is constant, intermittent, edge-triggered, or tied to a clipboard operation.
- Record whether each computer uses Wi‑Fi or Ethernet and save ping results from the time of failure.
- Note recent application, operating-system, VPN, firewall, or antivirus changes.
- Capture the screen layout and the logging level before performing a reset.
- For clipboard failures, specify text versus image, approximate payload size, direction of transfer, and whether pointer control remains responsive.
This information distinguishes a transport problem from a layout error or an operating-system integration issue without requiring you to disable multiple protections at once.
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Quick Recap
What not to assume
- A software KVM is not a video switch; the documented Deskflow model carries keyboard, mouse, and clipboard data over TCP without carrying display output.
- A normal ping does not rule out excessive logging, clipboard stalls, Handoff, or security software.
- Ethernet will not fix a wrong screen layout, cursor lock, or disabled clipboard setting.
- Synergy’s version numbers, UI labels, clipboard limits, and beta transfer behavior should not be generalized to Deskflow, Barrier, or other derivatives without checking their own documentation.
- There is no evidence here for a universal Wayland remedy or a universal latency threshold.
Source-backed references
- Synergy Help Center: What to do when the mouse cursor lags? (guidance checked October 7, 2026; includes Synergy 3.7.2 and beta-version clipboard details).
- Synergy Help Center: Basic troubleshooting steps (guidance checked October 3, 2026; includes Synergy 3.7.2 and Synergy 1.21.4 troubleshooting).
- Deskflow project (software-KVM architecture, TCP connection, and clipboard synchronization without video).
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