How SpeedFast Works

Why we measure more than just download speed

A speed test that only reports a single download number misses half the story. Real-world internet performance depends equally on latency (ping), latency stability (jitter), upload throughput, and responsiveness under load (bufferbloat). SpeedFast measures all four in a continuous test sequence that takes under 30 seconds.

For the formal mathematical algorithms and peer-review details, read our full [SpeedFast Measurement Methodology](/methodology).

Phase 1: Latency & Jitter (Idle)

Before any large data transfer begins, the test establishes a baseline. It sends a sequence of lightweight HTTP requests to the nearest edge server and records the round-trip time (RTT) for each.

Idle Ping is the lowest stable RTT recorded. [Jitter](/guides/what-is-jitter) is the average variation between consecutive ping measurements — a low jitter score means your connection is steady and predictable, which matters significantly for video calls and online gaming.

Phase 2: Download Throughput & Loaded Ping

To measure download speed, the test opens multiple parallel HTTP connections to the edge server and streams binary chunks. Using parallel connections is essential: a single TCP stream rarely saturates a modern broadband connection because TCP's congestion-avoidance algorithm ramps up gradually.

The first 1–2 seconds of throughput data are discarded as a "warm-up" period to let TCP connections reach steady-state. The reported download speed is a rolling average of sustained throughput across all parallel streams, not an instantaneous peak burst.

While data is actively downloading, the test continues sending ping requests in the background. The resulting measurement — Download-Loaded Ping — reveals whether your connection suffers from [Bufferbloat](/guides/what-is-bufferbloat), where large downloads create severe lag for other devices on your network.

Phase 3: Upload Throughput & Loaded Ping

Upload speed is measured by sending binary payloads to the edge server across parallel connections, again discarding the initial ramp-up to report sustained throughput.

Upload-Loaded Ping is measured simultaneously to check for upstream bufferbloat. For households with multiple people on video calls or working remotely, upstream bufferbloat is often the hidden culprit behind audio dropouts and frozen video.

Global Edge Infrastructure

SpeedFast runs on Cloudflare's global edge network, with measurement endpoints deployed across hundreds of cities worldwide. When you start a test, your browser connects directly to the topologically closest edge node, minimizing artificial routing delays that would skew results.

Browser-native, zero plugins

The entire test runs natively in your browser using standard Web APIs (Fetch, Web Workers, and High Resolution Time). No Flash, no Java, no browser extensions, and no background applications required. The test logic runs in a background Web Worker so that UI rendering never interferes with measurement timing.

Reviewed by Engineering

This testing methodology is maintained and continuously benchmarked by the SpeedFast Engineering Team. Learn more about our technical rationale in the [Measurement Methodology](/methodology) or explore our [Internet Speed Guides](/guides).