Specs: Intel 8400 duo core 3.3ghz EVGA GTX 465 Windows 7 64 bit 8gb of DDR 2 RAM 750w PSU Problem: It lags immensely when I step out into the outside world. Whenever that door opens the lag starts. I've played and beaten GTA before and when I came back to it, it does this. The only thing that has changed about my computer since I played it was the graphics card. I have no idea what is going on.
GTA 4 is poorly optimized. Try latest patch of GTA 4 works well and improves game's performance.