Thoughts on Apache Issue

We have been troubleshooting site slowness on a particular server. What is weird however is we have always used MPM event or MPM worker. But if we switch to MPM Prefork we are noticing it is performing 10s + faster on page load speed vs the other two. The busy site we tested with is usually around 35s on Gtmetrix but now its 19s. That a huge performance gain I would assume. We even installed Litespeed on the server which made no difference. Only switching to MPM Prefork seems to work. I really dont get it. Is this normal for shared hosting? or it just the tuning of MPM Event or Worker that could be at fault?