Is network replication only accessible by UClasses?

“A lot of math” - are we talking about 20 million operations per second here? Because otherwise your cpu most likely will be able to handle that with ease.

What you’re trying to do smells a lot like premature optimization.

Make working prototype first, using uclasses, and if it is slow, start optimizing.

https://wiki.unrealengine.com/Garbage_Collection_Overview

According to the wiki, anything that is not UObject-based will not be garbage collected. Which makes sense.