That's allot of changes 😄 (hope it doesn't cause to much back (s)lash )

Jokes aside I'm curious to see what the performance difference is here :)

Haha @sstok :D
This fixes only functions known that the PHP compiler optimizes and only those outside the global namespace. Without those two restrictions the diff would look even more insane (but even more performance gain might have gotten) :P
Sadly the tests are failing and there is no setup to test speed performance of a PR.
So, back to drawing board, lets see why I kicked over all the tests services ones again ;)

Failure is properly related, but you should be able to benchmark the performance gain using https://blackfire.io/ I remember @nicolas-grekas already tried to improve performance using this but the improvements were either to low or only made sense at some places 🤔

On VarDumper, I already measured a significant perf improvement when using root-namespaces for listed functions only. This does not apply to all functions, only to this specifically optimized list, see
PHP-CS-Fixer/PHP-CS-Fixer#3048

when applied on PHP CS Fixer, I got about 5.5% boost of fix command (under PHP 7.1.9)