March 19, 2025·10 min read

Benchmarking CDN Download Speeds

Fast CDN claims are easy to make and hard to verify. This guide covers measuring real download speed across CDNs, spotting regressions, and using sample files of varying sizes for meaningful benchmarks.

What speed actually means

Throughput — bytes per second once download is steady
Time to first byte (TTFB) — latency to start streaming
Total time — TTFB + throughput for the full file

A CDN with 50ms TTFB and 100MB/s throughput wins on small files. A CDN with 200ms TTFB and 500MB/s throughput wins on large files. Measure both.

curl one-liner for quick checks

curl -o /dev/null -w \
  "TTFB: %{time_starttransfer}s\nTotal: %{time_total}s\nSpeed: %{speed_download} B/s\n" \
  https://example.com/100mb.bin

Multi-region benchmark script (Node.js)

import { performance } from 'perf_hooks';

async function bench(url) {
  const start = performance.now();
  const res = await fetch(url);
  const firstByte = performance.now();
  const bytes = (await res.arrayBuffer()).byteLength;
  const end = performance.now();

  return {
    ttfbMs: firstByte - start,
    totalMs: end - start,
    throughputMBps: bytes / 1024 / 1024 / ((end - start) / 1000),
  };
}

const urls = [
  'https://cdn-a.example.com/100mb.bin',
  'https://cdn-b.example.com/100mb.bin',
];

for (const url of urls) {
  console.log(url, await bench(url));
}

Use the right test file size

TTFB dominates for files under 1MB. Throughput dominates above 10MB. Test both:

1MB — TTFB and early-byte characterization
10MB — steady-state throughput begins
100MB — realistic large-file benchmark
1GB — long-duration throughput, thermal throttling

Geography matters

Run benchmarks from multiple regions — ideally from your user distribution. Services like AWS Lambda in N. Virginia, Frankfurt, and Singapore give you 3-region coverage for free. Or use dedicated tools like Catchpoint or ThousandEyes.

Cold cache vs warm cache

# Cold: request a unique URL so cache can't hit
curl "https://cdn.example.com/100mb.bin?cb=${RANDOM}"

# Warm: hit the same URL twice, measure second run
curl -s -o /dev/null "https://cdn.example.com/100mb.bin"
curl -o /dev/null -w "%{speed_download}\n" "https://cdn.example.com/100mb.bin"

Cold-cache numbers tell you CDN origin performance. Warm-cache tells you edge performance. Both matter.

Benchmarks to track over time

p50 / p95 / p99 throughput from each region
TTFB trend week over week — regressions are almost always network changes
Error rate (5xx, timeouts) as a percentage
Bandwidth cost per GB served

Tools worth knowing

ab (Apache Bench) — simple concurrent load test
k6 — modern load testing in JavaScript
wrk — high-concurrency HTTP benchmarking
cdnperf.com — third-party CDN comparison

For upload-side performance, see resumable uploads with tus. For general large file handling, read large file upload guide.