Is that a laptop ? if yes, that's probably because of san andreas game's sort of compatibility problem.
In Turkey with my main computer (desktop) i was playing san andreas and samp with minimum 90 fps. And the desktop is like this :
Core 2 Duo @2.40Ghz
3 gb ram
1gb ati hd4650
320 gb hdd
Then, i was going to go to France, so dad bought me an awesum laptop, i could play any game i want, even gta 4 with average graphics (like, normal but not high or very high). The laptop's specs below :
Core i7-3630C CPU @2.40Ghz
8 gb ram
2 gb Nvidia GeForce GT 650M
1 tb hdd
And can you imagine how much fps do i get in san andreas ? Just 40. Maximum 70 if i disable everything and reduce the resolution to the lowest. What's more, i can still play any other games damn smoothly. This lag thing happens only in san andreas, and i don't know why. This game gotta have something to do with laptops.