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Start with a repeatable baseline
Use the same long page, renderer version, machine, network conditions, and output path for every trial. Warm and cold runs can differ, so record both when startup or caching may matter.
- Total wall-clock time, including process startup.
- Output PDF size and page count.
- Number of images, their pixel dimensions, and whether each is remote or local.
- Whether JavaScript creates content or loads images.
- Peak memory and CPU, if your operating system exposes them.
- Visual checks: missing images, layout shifts, clipped content, incorrect fonts, and altered sharpness.
Run at least several identical conversions before changing anything and keep a table of results. A faster file is not an improvement if dynamic content disappears or print-quality images become unusable.
A minimal timing harness
/usr/bin/time -v wkhtmltopdf --enable-javascript https://example.com/long-page baseline.pdfFor a local HTML file, use an absolute file:// URL or the path format accepted by your platform. If the page requires authentication, reproduce the same headers or cookies in each run.
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Why wkhtmltopdf slows down on image-heavy pages
Qt WebKit must load the document, execute scripts, fetch and decode media, lay out every page, and encode the PDF. A long page multiplies layout and pagination work; many large images increase network, decoding, memory, and output-encoding work. A remote image server, a failed request that waits for a timeout, or a script that never reaches a stable state can dominate the total even when PDF writing itself is quick.
Reduce image work without hiding required content
Resize source files before rendering
Prepare each image near the largest physical size it will occupy in the PDF. Supplying a 6,000-pixel photograph for a 500-pixel slot forces needless download and decode work. Resize with an image pipeline before invoking wkhtmltopdf, retain the original for other uses, and compare fine text and edges in the resulting PDF.
Test image DPI and JPEG quality
The wkhtmltopdf manual documents a default --image-dpi of 600 and a default JPEG --image-quality of 94. These are output controls, not promised speedups. Test a lower DPI and a lower quality that still meets your print requirement:
wkhtmltopdf --image-dpi 300 --image-quality 85 input.html tuned.pdfUse --lowquality when a smaller, lower-quality PDF is acceptable. Inspect photographs, diagrams, and small labels at their intended viewing size. Do not assume lower quality always makes rendering faster; benchmark your actual document.
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Remove images only when they are genuinely optional
--no-images prevents image loading. It can be useful for a text-only export or a diagnostic run that answers “are images the bottleneck?” It is not a safe production optimization when images are part of the document.
Separate network loading from rendering
Make a copy of the page with all required images available locally and compare it with the remote version. A large difference points to DNS, TLS, server latency, rate limits, redirects, or failed requests rather than PDF layout. Also check whether image URLs return an error page, require a cookie, or are blocked by authentication.
wkhtmltopdf exposes image-loading and media-error behavior through command-line and library settings. In a library configuration, the corresponding controls include web.loadImages and load-error handling. Use those settings to create a diagnostic run, then restore strict failure handling for production so missing media is visible instead of silently accepted.
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Practical network checks
- Log each image URL and HTTP status before conversion.
- Download remote assets once into a temporary directory and render from local paths.
- Use deterministic asset versions so a cache or changing image cannot invalidate comparisons.
- Set sensible network timeouts outside the renderer and fail clearly when an asset cannot be retrieved.
Control JavaScript and waiting
JavaScript is enabled by default in the wkhtmltopdf command-line manual, and the documented --javascript-delay default is 200 ms. The library settings expose web.enableJavascript, load.jsdelay, and load.stopSlowScript.
When disabling JavaScript is safe
Try:
wkhtmltopdf --disable-javascript input.html no-js.pdfOnly keep this change if the HTML already contains all text and images needed in the PDF. Client-rendered applications, lazy-loaded images, charts, and authentication flows may produce an incomplete document without scripts.
Shorten an unnecessary delay
If your page is ready immediately, test a smaller delay:
wkhtmltopdf --javascript-delay 50 input.html short-wait.pdfA delay is not a readiness detector. If scripts still fetch images or build the DOM after 50 ms, the PDF can be missing content. Prefer a page that signals readiness deterministically, and verify that every required selector and image is present before accepting the output.
Handle runaway scripts
Use the slow-script control in library settings when a script can loop or block. Treat a stopped script as a possible correctness failure, not merely a successful speed improvement; record an error and inspect the PDF.
Batch conversions and process startup
If one long page is slow, changing process startup will not materially alter its render time. If you convert many documents, repeated launches can become significant. The manual documents --read-args-from-stdin: each input line is processed as a separate invocation with its arguments combined with the main command.
printf '%sn'
'https://example.com/a.pdf a.pdf'
'https://example.com/b.pdf b.pdf'
| wkhtmltopdf --read-args-from-stdinAdapt quoting for URLs containing spaces or shell metacharacters. Benchmark a batch both with and without this mode. It targets launch overhead, not image decoding or page layout, and it does not make a single conversion intrinsically faster.
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Do not treat smart shrinking as a speed switch
Smart shrinking is enabled by default and changes the pixel-to-DPI relationship used for layout. It is a layout control. Test --disable-smart-shrinking only when scaling, clipping, or pagination is wrong, and compare output dimensions and page breaks. The documentation does not establish a general rendering-speed gain from either setting.
PDFKit requires a different diagnosis
PDFKit creates PDF objects in your application rather than rendering a webpage through a browser engine. Profile the stages separately:
- Image reads from disk, object storage, or HTTP.
- Image decoding and any resizing or format conversion.
- PDFKit drawing and document generation.
- Backpressure, filesystem writes, and the destination stream.
Use a monotonic timer around each stage and capture memory usage. A slow HTTP request should be fixed at the fetch layer; a huge decoded bitmap should be resized before insertion; a blocked output stream requires stream handling rather than renderer flags.
Understand PDFKit page buffering
PDFKit normally flushes pages to the output as new pages are created. Setting bufferPages: true lets code revisit earlier pages and is documented for versions 0.7.0 and later. It is intended for tasks such as adding page numbers, not as a measured speed optimization. Buffering can increase memory pressure on long documents.
const PDFDocument = require('pdfkit');
const fs = require('node:fs');
const doc = new PDFDocument({ bufferPages: true });
doc.pipe(fs.createWriteStream('report.pdf'));
// Add pages and images here.
const range = doc.bufferedPageRange();
for (let i = range.start; i < range.start + range.count; i++) {
doc.switchToPage(i);
doc.fontSize(9).text(`Page ${i + 1}`, 50, 760);
}
doc.end();For maximum throughput, avoid buffering unless you need to edit earlier pages. Stream output and monitor the destination’s drain behavior instead of accumulating the entire PDF in memory.
A safe optimization workflow
- Capture a baseline and save the PDF.
- Classify the bottleneck: network, JavaScript/readiness, image decode, layout, encoding, startup, or output I/O.
- Change one variable: resized assets, local assets, DPI, quality, JavaScript, delay, or batch mode.
- Run the same timing protocol.
- Check every page for missing or shifted content and compare file size and image quality.
- Keep the change only when its trade-off fits your requirements; otherwise revert it.
Troubleshooting common failures
Images are missing
Check URL status, redirects, TLS, authentication, relative paths, and file permissions. Render with local copies to isolate network access. If disabling JavaScript caused the failure, restore scripts or make image loading server-side.
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The delay may be too short, JavaScript may be disabled, or a slow script may have been stopped. Increase the wait temporarily, add a deterministic readiness condition in the page, and compare the DOM content that exists at capture time.
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Conversion hangs or times out
Look for an unreachable asset, an endless script, a stalled response, or a page waiting on an external service. Test a local asset set, enforce request timeouts, and use slow-script handling. Do not merely increase the timeout without finding the blocker.
Output is smaller but unreadable
Restore DPI or JPEG quality incrementally. Resize images to their actual print dimensions rather than compressing everything aggressively.
Batch mode produces incorrect filenames
Each stdin line is parsed as arguments. Quote paths and URLs correctly, test a two-document batch, and use explicit output names with characters safe for your shell.
User content creates a security risk
The wkhtmltopdf project warns not to use it with untrusted HTML unless user-supplied HTML and JavaScript are sanitized; otherwise the renderer can expose the server to a complete takeover. Sandbox conversion, restrict network access, validate URLs, and treat this security work as separate from performance tuning.
Or skip the browser setup
If your goal is a clean screenshot or PDF rather than reproducing a local wkhtmltopdf pipeline, ScreenshotNeo provides a single request to its website screenshot API. It accepts cookie and consent banners before capture and removes more than 60 known consent platforms, newsletter popups, and chat widgets. Bot checks, blank pages, failed loads, timeouts, and cache hits are not billed, and response headers identify the page verdict and billing status.
cURL:
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpPython:
import requests
r = requests.get("https://api.screenshotneo.com/v1/shot", params={"access_key": "YOUR_API_KEY", "url": "https://stripe.com"}, timeout=90)
open("shot.webp", "wb").write(r.content)Node.js:
const q = new URLSearchParams({ access_key: 'YOUR_API_KEY', url: 'https://stripe.com' });
const res = await fetch(`https://api.screenshotneo.com/v1/shot?${q}`);See the complete parameter reference in the ScreenshotNeo documentation. Options include full-page capture with lazy images loaded, CSS-selector element capture, device and viewport control, retina scale, PDF paper and margin settings, custom CSS and JavaScript, waits, request blocking, headers, cookies, timezone, geolocation, resizing, chosen cache TTL, signed image links, asynchronous webhooks, bulk capture of up to 100 URLs per call, usage data, and an MCP server with take_screenshot, get_page_info, and capture_pdf for AI agents.
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Frequently Asked Questions
Can I make one very long wkhtmltopdf page fast just by adding more CPU?
More CPU may help only when profiling shows CPU saturation. It will not fix slow remote requests, blocked JavaScript, oversized images, or a stalled output stream.
Should I convert every image to JPEG?
No. Choose a format appropriate to the artwork and test readability. JPEG quality controls apply to JPEG compression; converting line art or transparency-heavy images can reduce fidelity.
Is PDFKit a drop-in replacement for wkhtmltopdf?
No. wkhtmltopdf renders HTML/CSS through Qt WebKit, while PDFKit requires you to construct PDF content in code. Migrating usually means rebuilding layout and pagination logic.
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