The support suggested that I switch to MPM Worker to speed up the site. Which I did but, if anything, it made it run slower. I switched back to MPM Prefork. Any particular reason why people believe that MPM Worker should be faster when I see the opposite?
It depends on what your complete setup is, including Apache and PHP versions, PHP setup, memory, CPU cores, etc. Switching to a different MPM means a completely different configuration setup.
As of now everything seems to work nice (fast). But if I do decided to rebuild it back to MPM Worker (or Event) what other things should I tweak to make it work like it should? I usually go with the latest everything (PHP, Apache, etc.).