Mike Bailey

Latency is a Killer

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If you’re hosting your website overseas you might want to rethink. I’ve been doing some research on the true impact of hosting Australian websites in the U.S. and it’s worse than you might think.

I plotted the time it took to retrieve 200 files from 1-200KB from two locations. The first was a server in the same city as me (kindly provided by SCT.com.au) and the second is with Slicehost in the US. We’re comparing the effects of 15ms RTT vs. 250ms RTT on performance.

You might imagine the US server would simply add 250ms to the total download time. The reality is that it can take ten times longer to get files from the U.S. The TCP protocol includes provisions to avoid network congestion that were designed before we had broadband. I discussed these in my presentation ‘Speed Matters’ (slides here).

The Effects of Latency on Load Time

The stepping you see is due to TCP Congestion Control (RFC 2581) and Delayed ACK (RFC 813).

My investigation was inspired by a talk by John Rauser at VelocityConf last month.