[VEN]

In the whole it's a complicated question. For example with the short functions like fm_set_user_gravity FMU is the winner over FUN because efficiency is the same but FUN has an internal forward hooks for some of his natives which slows module down. The same goes for Cstrike. It may be not a bad idea to use FM instead of Cstrike to change some of the player private properties like money, deaths, ammo etc. But for most coders it doesn't worth to port such natives to the FM.

By disabling modules you can't optimize your script. You can increase AMXX runtime performance. But again if you are not understand what would be more efficient in the different situations you probably shouldn't blindly use FM instead of bunch of other modules. So Hawk's "hint" was reasoned.

Also not the last role play the fact of how often you call your function. So for example if you create some armoury entities only at the map startup, you shouldn't bother that fm_set_kvd is a bit slover than DispatchSpawn.

In the most cases using FMU will not hit the performance so you shouldn't bother, but you should also undersant what you are doing.