One thing that you could have mentioned here is modifications to your GTA:SA game folder in general - some weapon skin mods are usually not that demanding, but modding all of your vehicles, skins and weapons may cause frame rate issues, or the stream memory problem.
If you are planning to install any major graphical overhauls, or ENB, on a computer that may already have issues with running SA:MP, or GTA:SA, or it may not have a dedicated graphics card, I'd highly advise against installing it. If you have a computer that suffers with ENB enabled, you could modify your ENB .ini file to run better as there are several different videos on YouTube that display ENB with lower settings, and contain a complete list of the settings you require to edit your file, and eventually attain this. Then again, even if you suffer with an optimised ENB (which has settings designed for a low end PC), you'll still suffer from some sort of frame rate loss - you can therefore just use the hotkey to turn it on and off, and perhaps turn it on if you're taking any screenshots.
And, of course, the best way to avoid any FPS-related issues is to not install any modifications (graphical) at all.