A NAS backup can make Plex buffer when the two workloads compete for the same disks, processor time, or network link. But buffering during a backup does not identify the cause by itself: Plex may be struggling to transcode, the storage pool may be busy, or a shared network path may be saturated. Compare playback with the backup running and paused, while checking which resource is actually under pressure.
Why can Plex buffer when a NAS is backing up?
Plex needs to read media from storage and deliver it to a client; a backup may read, write, scan, or process data at the same time. If those activities overlap on a constrained resource, Plex can fall behind. The effect depends on the NAS, backup type, storage layout, playback mode, and network path—not simply on whether a backup is active.
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Plex identifies processor power, network speed, and storage space as general constraints. For local playback, network speed is usually less likely to be the limit than it is for remote viewing. Storage activity can still matter, especially when media and backup operations use the same disks or array.
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| Resource | Where contention can occur | Clues to check |
|---|---|---|
| Disk I/O and storage | Media reads and backup reads or writes compete on the same disks, pool, or volume; maintenance operations can add activity. | High disk utilization or latency, a busy storage pool, drive-health warnings, or playback that improves when the backup pauses. |
| CPU | Plex transcoding and other NAS tasks compete for processor time. | High CPU use during a transcode; Plex’s transcode speed falls below real time. |
| Network | Remote backup and remote Plex playback share limited internet upload, or local traffic shares a constrained LAN path. | Network traffic approaches the capacity of the relevant link while playback or backup is active. |
Disk I/O: the storage path is busy
When media and backup activity use the same storage, their reads, writes, and metadata work can compete. The precise impact varies by workload and system. A backup to a different destination does not automatically eliminate contention: the NAS may still need to read the same source files, and the destination or network path may be shared.
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Backup types should not be treated as interchangeable. Synology’s DSM 6.2-era Managing Performance guide recommends scheduling Active Backup and Hyper Backup off-peak. It distinguishes snapshot creation, which it says does not use significant I/O, from snapshot rotation, which may require more I/O. Those are vendor guidelines for that guide’s context, not a guarantee about every NAS, backup product, or current firmware.
If disks are persistently busy or unhealthy, investigate the storage system rather than assuming Plex itself is at fault. Synology’s Hyper Backup troubleshooting guidance points users to disk utilization and health, storage-pool operations such as repair or RAID changes, CPU processes, and volume fullness. It says freeing space may help backup performance when volume use exceeds 90%; that is not a universal threshold for Plex buffering.
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CPU: Plex may be transcoding
Plex playback can use substantial CPU when the server must transcode media for a client’s format or requested quality. Subtitles that need to be burned into the picture can also require transcoding. Plex says, “The majority of your ongoing processor (CPU) usage when using Plex will come from any transcodes for content that are needed.” It also warns that some NAS processors cannot handle certain transcoding workloads, and some low-power models cannot transcode at all. A backup might add CPU work, but that does not mean every backup task will saturate the processor.
Check whether the session is Direct Playing, Direct Streaming, or transcoding. Plex’s buffering guidance says transcode speed should be at least real time (1.0) to keep up; an average around 1.3 or higher provides headroom in its diagnostic context. These are troubleshooting indicators, not guaranteed performance specifications for every NAS, file, or client.
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Transcoding also uses temporary storage. Plex’s transcoding overview describes that temporary use, while its Transcoder documentation covers the storage location and related settings. The temporary directory needs adequate free space; Plex warns against putting it on a network share or disk. A fast local location may help if temporary transcode storage is the problem, but it does not automatically resolve contention on the media array.
Network: distinguish local traffic from remote upload
Plex says local network speed is generally less important for local viewing than remote playback, where server-side internet upload is often the constraint. A cloud backup or backup to a remote NAS can use that same upload capacity; remote Plex streams may then compete with it. A local backup can instead load the LAN or a shared switch path, depending on the layout. Before changing network equipment or limiting bandwidth, identify the link both workloads actually use and check its traffic.
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How to check whether the backup is slowing Plex
- Make playback repeatable. Use the same title, client, quality setting, and time window. Note whether Plex reports Direct Play, Direct Stream, or a transcode.
- Compare two conditions. Observe playback while the backup runs, then pause the backup and repeat the same playback. This helps establish whether the slowdown is associated with the overlap, though it does not by itself prove which resource caused it.
- Watch the NAS during both periods. Check CPU processes, disk utilization or latency if available, network traffic, storage-pool operations, and drive health. On Synology DSM, Resource Monitor provides CPU and disk views; labels can vary by DSM version.
- Match the evidence to the resource. If CPU is saturated while Plex transcodes, check client compatibility, subtitles, requested quality, and the NAS’s transcoding capability. If disks are heavily utilized or unhealthy, inspect the volume, pool, and drives. If a network link is saturated, identify whether the bottleneck is internet upload or local traffic.
- Change one thing and retest. Schedule routine backups off-peak, then repeat the playback comparison. If measurements show network contention and the backup software supports it, reducing backup concurrency or separating traffic may help; there is no universal setting that fits every NAS.
What fixes are worth trying?
Move routine backups off-peak
Scheduling is a reversible first step when playback and backup overlap. Synology’s DSM 6.2-era performance guide advises setting scheduled Active Backup and Hyper Backup tasks to off-peak hours to minimize their impact on day-to-day activity. Recheck Plex after changing the schedule to see whether the overlap was relevant.
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Address the measured cause, not the symptom alone
- Transcode bottleneck: try a client-compatible format or playback quality that avoids unnecessary transcoding, and check whether subtitles are forcing a transcode. Confirm that the NAS can perform the required transcode.
- Storage issue: investigate disk health, utilization, pool activity, and free volume space. Replace a drive only if checks identify a failing or unsuitable drive; a larger drive by itself is not a general fix for contention.
- Network contention: confirm the congested link and the traffic using it before changing bandwidth settings, network gear, or backup concurrency.
How much can a NAS backup slow Plex?
There is no universal percentage established for how much backups slow Plex. The result depends on the NAS and its configuration, backup workload, whether Plex is transcoding, and which disks or network links are shared.
Synology reported a 95% improvement over the previous release in a 2024 Hyper Backup lab test using 10 million files totaling 18 TB with a 5% file-change rate. That is a vendor-reported backup-throughput comparison for a particular version and workload—not a measurement of Plex playback or backup-related buffering. See Synology’s account of the test.
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