Eh, it’s been fast in the real world for quite a while. I think it’s due to two main things:

1) Startup time had to be fast because of how it worked in its most common use case (yes, yes, FPM, but still) which tends to encourage or require making everything fast.

2) Writing C modules for PHP is relatively easy and it has a culture of anything halfway important being in C, going back to the early days of the language. The answer for how to be really fast with real-world workloads in almost every scripting language is basically “get out of it and into C ASAP” but PHP’s culture embraces that more than most.