no one can really help you cos it could be anything..from dirty cpu fan to you having 58 different mods installed which *surprised pikachu face* lag your game
if your pc is healthy, and other games work fine, then you should start checking what's wrong with gtasa..
if they don't, there's -something- wrong with your computer and it's on you to figure it out..no one can help you with it, since it can be anything..for me (at one point) it was nearly-full hdd and freeing 100gigs made it fine again.
fpslimit command won't magically give you fps so don't listen the dudes above..you turn it OFF, you get a recording software that shows you what fps you have, you play for a while and check what the numbers are..then you use that to limit the fps (with the fpslimit cmd) so you get smooth gameplay. If numbers on the screen spike from like 60 to 80, you set it to 60 so you get a nice non-twitchy gameplay. You can set it to something like 70, but then you will get fps drops and that might not be what you want. Using fpslimit 90 won't do anything since your pc can't reach that (in this example) number and you'll still be stuck on 60-80 depending on how many objects are around you.
lowering graphic settings, draw distance, disabling /dl..these are all stuff that would give you more fps but at the cost of normal looking gameplay..mess with it for a while and find the middle ground, so you have decent fps with still a decent looking game.
mods that give you fancy cars, weapons, buildings etc lag your game..if your pc can't handle it then just remove them.
if you want to "boost" the fps by using mods there is ton of them around which remove textures/effects/sounds therefore give you more fps..google it and abuse the fact no one said what's allowed and what's not allowed in here (there is a topic on this subject made by former cm, but not even admins stand behind it so forget it even exists).