Tim Bray - Wikipedia Notes:

“As of today, there are 124 servers, fairly heterogeneous, although these days they’ve pretty well standardized on dual-Opteron boxes. The MediaWiki software is PHP-based, mostly running on Fedora; I wonder if this is the world’s largest-scale PHP deployment, or would Yahoo top that? They get a pretty good hit rate on their Squid caches; basically, the whole system scales about linearly with the number of servers they deploy.

[…] Having said that, speaking both as a user and contributor, I find that Wikipedia’s performance is mostly pretty terrible; usable, but irritatingly slow. So there’s certainly room for improvement.”