The wikipedia list is always both interesting and depressing to me. Recently, I've learned that:
PHP reboots on each request; that is, there's apparently(?) no way to keep a long-running process around to process requests faster. This means that one way to improve per-request latency is to cut down on the total boot time of your application!
Rendering a page (this is not including boot-up or fetching page content(?)) takes 800ms. That's almost a second! Now I sorta understand why they came up with that kooky memcache variant -- at those speeds, even loading data from disk is relatively quick.
On the other hand, they're past 10khits per second.
Wikipedia's tech is fascinating to me because they've clearly got decently smart people working on it but it seems like early design decisions hamstrung later ones. There's a lesson there, I think: something about how the second system syndrome isn't as bad if you can get to it early.