fps and gameplay in FPSC are complex issues. You need a fast and powerful system to get the best from it, If you can get hold of the best on the market thats helpful, but that alone is not enough.
There are issues inside the FPSC engine which make it so.
With skill at learning the software, much hard work and effort in level design and optimisation you can maintain reasonable frame rates.
The serious lagg issue however is another matter its not system or user controllable or affectable and is a problem issue or bug internal to the engine and can only be partially improved by trial and error in you level building.
For some levels serious lagg issue may be impossible to remove and will exist in certain restricted level areas no matter what you do.
That is until I or someone else can disprove this by defeating the issue. As far as I amware TGC cant overcome it so I doubt users will - other than by some act of fate or luck.