Make sure you download it from :
http://sa-mp.com/download.phpDownload the one from SA-MP 0.3c Client Mirror by SA-MP Team
If you had no lag in 0.3b , but now in 0.3c you have.. It's not your Computer problem. It's a problem with the SA:MP patch. Unfortunately you have to wait for the next SA:MP Patch.
Another thing you could try is - uninstall all the mods. Or comletely re-install the whole San Andreas and SA:MP (Uninstall San Andreas , Uninstall SA:MP , Delete GTA San Andreas User Files from My Documents , delete everything related to GTA SA from C:\Users\YourUser\AppData) And after that , install San Andreas and SA:MP SA-MP 0.3c Client Mirror by SA-MP Team .
I think the lag is caused by the new objects/items added to 0.3c..
Another thing that can cause lag - Press ESC , Go to Options , Display Setup , Advanced , FRAME LIMITER. Make sure it's off. After you done that , go back to game and type ''/fpslimit 100'' . Normally the limit is set on 20.
Try changing your Anti - Aliasing to 1. Resolution lower , Visual FX quality lower. Try 16 colour setting. You can do it by chaning resolution to 1111x1111x16.
To see your FPS , press NUM *