> The PHP runtime is incredibly complex

Say this to the team behind Ladybird browser: https://awesomekling.substack.com/p/how-were-building-a-brow...

They have the benefit of a project leader with many years of browser implementation experience AND much lower performance goals.

(Items 5 and 3 on the list in the linked page.)

80/20, also they're not implementing JIT and such because of complexity.