Why crypto arbitrage windows close before your REST poll completes

TL;DR : Crypto arbitrage windows on liquid pairs now close in under 100 ms. A REST polling loop typically takes 1–1.5 seconds round-trip. WebSocket delivers the same data in 20–100 ms. If you're still polling REST endpoints for orderbook data in 2026, you're missing the majority of opportunities — not because your strategy is wrong, but because your data plane is fundamentally too slow. This post walks through the math, shows a benchmark I ran on a handful of major exchanges, and provides production-grade Python code for a WebSocket client that handles reconnects, heartbeats, and orderbook reconstruction. 1. The numbers that broke REST polling When I started writing crypto arbitrage bots a few years ago, polling Binance's REST API every 500 ms was perfectly acceptable. Spreads were wide, arbitrage windows lasted multiple seconds, and the orderbook for BTCUSDT moved slowly enough that a half-second-old snapshot was still tradeable. In 2026, the same approach doesn't work. Here are the numbers as they stand today: Metric Value Median crypto arbitrage window on liquid pairs 30–80 ms Window closes in under 100 ms ~90% of cases REST round-trip latency (request → response → JSON parse) 1.0–1.5 seconds WebSocket update delivery latency (push from exchange to client) 20–100 ms The math is brutal. A 100 ms window cannot be caught by a 1500 ms poll. By the time your REST response arrives, the orderbook you're reading is 15 cycles stale. You're not "slow" — you're not even in the same temporal universe as the event you're trying to react to. 2. Why REST is fundamentally slow REST APIs over HTTPS carry overhead that adds up: TCP handshake — three packets to establish, typically 50–150 ms on intercontinental hops. TLS handshake — another full round-trip, 30–100 ms. HTTP request/response — the actual data exchange. JSON parse — depending on payload size, 5–50 ms. Rate-limit budget — most exchanges cap REST to 10–20 requests per second per IP. Polling faster gets you banned. Yes,