Chapter 1 of High Performance Browser Networking · Read the chapter
The Many Components of Latency
Latency is the time it takes for a message or packet to travel from the origin point to the destination.Speed of Light and Propagation Latency
Light in fiber travels at roughly 200,000,000 m/s — about 1.5× slower than in a vacuum, because the refractive index of glass is ~1.5.
That 42 ms is the floor. No protocol, no server, no amount of money removes it.
Last-Mile Latency
The largest latency typically occurs in the “last mile” — from home/office to your ISP’s first routing node — not on transoceanic routes. Reason: ISPs must run cables through residential areas, consolidate signals, then route to core nodes. These first few hops consume dozens of milliseconds.Bandwidth in Core Networks
A single optical fiber carries many wavelengths at once through wavelength-division multiplexing (WDM), so total link bandwidth is per-channel rate × number of channels. As of 2010, 400+ wavelengths at 171 Gbit/s each — over 70 Tbit/s on one fiber. Matching that with copper would take thousands of links. The backbone is not the bottleneck.Bandwidth at the Network Edge
Your available bandwidth is the capacity of the lowest-capacity link between client and server — and that link is almost always at the edge, not the core.
Akamai, Q3 2015, excluding mobile carriers (mobile is slower and far more variable).
For scale: HD video needs 2–10 Mbps depending on codec and resolution. The global average barely covers one stream.
A speed test measures the hop to your ISP, not end-to-end performance. Congestion at any intermediate node changes the answer.
Bandwidth Scales, Latency Does Not
Shortening the distance is the only real lever on latency, and laying new cable is constrained by terrain, politics, and cost.
Bandwidth is an economics problem; latency is a physics problem.So optimize the application, not the wire: cut round trips, move data closer to the client (CDN), and hide latency with caching and pre-fetching.