I replaced Puppeteer with a Zig library and went from 18 to 17,000 PDFs per second

John Soto — Wed, 15 Apr 2026 01:40:21 +0000

If you've worked with designers, you know the routine.

They hand you PDF templates: invoices, reports, certificates. Specific fonts, aligned columns, logos.

You look at it and think: "I could build this in HTML and CSS in 20 minutes."

So you do. Puppeteer. page.pdf(). Perfect output.

Then reality hits.

A client clicks "Download All" on 5,000 invoices and your server melts.

I optimized everything I could. Browser warmup, page reuse, connection pooling. Best I got was about 25 PDFs per second.

At some point you realize: you can't optimize around running a full browser engine to generate a static document.

So I tried something different

I'm a Bun user, and Bun is fast partly because it's built in Zig. I'd been wanting to learn Zig, and a PDF renderer felt like the perfect project: real problem, clear spec, and a chance to see what the language could actually do.

So I started building. Just a <Page> and a <Box>. If I could render that to a valid PDF, I had something.

Then I kept going. Styles, text wrapping, fonts with subsetting, images, QR codes. Each feature was a small, solvable problem. I paired with Claude, not to generate code blindly, but to work through the hard parts: the PDF spec, font parsing, AES encryption, layout edge cases.

The result

import { render } from "@slothpdf/render";

const template = `
  <Page size="A4" margin="20mm">
    <Box class="text-2xl font-bold mb-4">{name}</Box>
    <Box class="text-sm text-gray-600">{description}</Box>
  </Page>
`;

await Bun.write("invoice.pdf", render(template, {
  name: "John",
  description: "Hello World"
}));

No browser. No runtime overhead. Just template in, PDF out.

// Batch
for (const { index, buffer } of render(template, rows)) {
  await Bun.write(`invoices/${index}.pdf`, buffer);
}

// Merge into one multi-page PDF
render(template, rows, { merge: true })

// ZIP archive
render(template, rows, { zip: true })

// AES-256 encryption
render(template, data, { password: "secret" })

All options stack. Not a full HTML/CSS engine. Intentionally limited to predictable layouts.

Benchmarks

Apple M4, single-threaded, 1,000 unique invoices with custom fonts and 3-10 line items per invoice:

Tool	Speed
SlothPDF	17,000 /sec
jsPDF	7,750 /sec
Puppeteer	~18 /sec

Memory stays flat. 50,000 renders, no growth. 1,000 PDFs in ~60ms on a single thread.

Why it's fast

There's no trick. It's just fewer layers:

No browser, no DOM, no CSS parser
Fonts are parsed once and cached for the process lifetime
Templates are parsed once, only data binding and layout run per render
Memory is allocated in arenas that reset after each PDF, no GC
The Zig engine writes PDF bytes directly, no intermediate format

Try it

bun add @slothpdf/render

Playground (build templates in the browser)
npm
GitHub

Requires Bun 1.1+. macOS and Linux.