alt Hacker News>Do I understand why they're so much lower latency than REST calls on mobile networks? Not really: In theory, it's still a round-trip but for some reason an open connection can pass data through an order of magnitude (or more) lower latency on something like a 5G connection.
It's because a TLS handshake takes more than one roundtrip to complete. Keeping the connection open means the handshake needs to be done only once, instead of over and over again.
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Yes and no: There's still a rather large latency improvement even when you're using plain HTTP (not that you should go without encryption).
I was very curious so I asked AI to explain why websockets would have such lower latency than regular HTTP and it gave some (uncited, but logical) reasons:
Once a WebSocket is open, each message avoids several sources of delay that an HTTP request can hit—especially on mobile. The big wins are skipping connection setup and radio wakeups, not shaving a few header bytes.
Why WebSocket “ping/pong” often beats HTTP GET /ping on mobile
No connection setup on the hot path
HTTP (worst case): DNS + TCP 3‑way handshake + TLS handshake (HTTPS) before you can send the request. On mobile RTTs (60–200+ ms), that’s 1–3 extra RTTs, i.e., 100–500+ ms just to get started.
HTTP with keep‑alive/H2/H3: Better (no new TCP/TLS), but pools can be empty or closed by OS/radios/idle timers, so you still pay setup sometimes.
WebSocket: You pay the TCP+TLS+Upgrade once. After that, a ping is just one round trip on an already‑open connection.
Mobile radio state promotions
Cellular modems drop to low‑power states when idle. A fresh HTTP request can force an RRC “promotion” from idle to connected, adding tens to hundreds of ms.
A long‑lived WebSocket with periodic keepalives tends to keep the radio in a faster state or makes promotion more likely to already be done, so your message departs immediately.
Trade‑off: keeping the radio “warm” costs battery; most realtime apps tune keepalive intervals to balance latency vs power.
Fewer app/stack layers per message
HTTP request path: request line + headers (often cookies, auth), routing/middleware, logging, etc. Even with HTTP/2 header compression, the server still parses and runs more machinery.
WebSocket after upgrade: tiny frame parsing (client→server frames are 2‑byte header + 4‑byte mask + payload), often handled in a lightweight event loop. Much less per‑message work.
No extra round trips from CORS preflight
A simple GET usually avoids preflight, but if you add non‑safelisted headers (e.g., Authorization) the browser will first send an OPTIONS request. That’s an extra RTT before your GET.
WebSocket doesn’t use CORS preflights; the Upgrade carries an Origin header that servers can validate.
Warm path effects
Persistent connections retain congestion window and NAT/firewall state, reducing first‑packet delays and occasional SYN drops that new HTTP connections can encounter on mobile networks.
Handshake cost: TLS adds 1–2 RTTs (TLS 1.3 is 1‑RTT; 0‑RTT is possible but niche). If you open and close HTTP connections frequently, you keep paying this. A WebSocket pays it once, then amortizes it over many messages.
After the connection is up, the per‑message crypto cost is small compared to network RTT; the latency advantage mainly comes from avoiding repeated handshakes.
For tiny messages, both HTTP and WS fit in one MTU. The few hundred extra bytes of HTTP headers rarely change latency meaningfully on mobile; the dominant factor is extra round trips (connection setup, preflight) and radio state.
If your HTTP requests reuse an existing HTTP/2 or HTTP/3 connection, have no preflight, and the radio is already in a connected state, a minimal GET /ping and a WS ping/pong both take roughly one network RTT. In that best case, latencies can be similar.
In real mobile conditions, the chances of hitting at least one of the slow paths above are high, so WebSocket usually looks faster and more consistent.
doesn’t HTTP keep connections open?