Big projects loading slowly - How to manage ?

I have seen a few ReadyAPI composite projects which are very big and load very slowly (about 15 minutes or so). I have some questions about this.

1 - How do we figure out the exact cause of the problem?
2 - Are there any limits to project size in terms of MB of size, number of tests etc., after which the project becomes too slow?
3 - If size is the cause of the problem, then do we just need to split the project into smaller projects?

Thanks.