Having multicore off means the CPU is only going to run the game on a single core of your CPU, if it is lagging I would suggest turning Multicore back to 1.
Q: My mouse isn't working ingame and the camera doesn't move and/or my game freezes when pressing Esc or Minimizing (Vista/W7)
Possible Solution:1. Press the Start Button (the round Windows one) and type "services"
2. If it asks you for permission as Administrator, press Continue
3. In the Services list, search for "Windows Event Log".
4. Double-click it, so another window opens.
5. Set "Start up Type" to disabled. Now restart your computer/laptop.
6. Once back, you will notice GTA San Andreas doesn't crash anymore or freezes your mouse!
Source:
Here