@Modz4Me mods folder don't decrease fps as was wrote here but the mods itself can if they are made wrong, mostly the scripts can do that. Using a lot of highpoly models and high resolution textures may just cause slower loading of some textures, but we are talking about more than 5-10-20 mods to do this.
I'm not 100% but some mods change each weapon individually to make muzzle velocity or the force of the round less which will have a bit more of a realistic effect. I think one was Ripplers Realism (https://www.gta5-mods.com/weapons/ripplers-realism) but I'm not for sure. So at least some people have the right idea.