Can you explain why you feel speed is a particular problem with PHP compared to other web languages? Pointing out Facebook's improvements/hacks isn't very relevant considering they are one of the highest-traffic sites on the entire internet. Most Java container servers have simply never been used for anything approaching the traffic Facebook deals with. And even then, Facebook is just optimizing to cut costs (reduce total server count+power usage), not to make their site load faster for users. It's trivial to add servers to a cluster, and the only real downside is cost, not speed.
The real bottleneck you run into LONG before hitting application server problems is your datastore. That's the one that requires real thought and hard work to deal with once you've pushed "buy a bigger database server" to its limits. Your failure to mention this or acknowledge how little application speed matters makes me wonder how experienced you really are with high-traffic websites.
Looking at an 'average' website, the ones that are most consistently slow or downright bad are Java sites. Having seen this in development first-hand, it's because Java gives you more than enough rope to hang yourself with if you're a clueless off-shored developer (as so much java software is written by). Too many Java websites do a horrific job handling session scoping, cookies, caching (lets cache the whole database in the app layer and loop on it, right?), and so on. By comparison, your standard php, python, or ruby website will be "defaulted" to doing things the correct, scalable way, unless you go out of your way to screw things up. Ruby is the most immature of the 3 from what I've seen, but PHP and Python are great choices over any compiled language for the vast majority of websites. And with PHP, unlike any of those other languages, you already know it can scale to running the biggest site on the internet!