i'm writing this with an assumption that you have plenty of ram, 8 gb if i'm not mistaken.
1) get "dataram ramdisk", or other ramdisk utility, im just refering to this one as its easy to use.
2) install it, open, set size to ~1GB, unformatted space, no saving/loading. start it.
3) in windows disk manager format the whole ram disk to ntfs file system.
4) copy gta3.img from your installation into the ram disk.
5) rename gta3.img in your installation to something else, for backup.
6) open cmd.exe, write 'mklink /d "D:\program files (x86)\rockstar games\gta san andreas\models\gta3.img" "G:\gta3.img" '
(assuming your ram disc is set up to be G: and hard drive is D:)
now you should be able to access the whole big archive inside ram without accessing hdd directly.
if lag has gone then its hard drive issue, of not, then most likely gpu.
if you're not lazy, you can repeat the process for some other archives in there that you could suspect of being faulty.
to return back to normal just delete the links in installation, unmount the ram disk and rename backup archives to their normal names.