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Dual-Core CPU Issues
If you run a dual core CPU, such as a Pentium D or AMD X2, or even a Pentium 4 with HyperThreading, you may notice that the game is quite jerky even at high framerates. Things seem to run fast and slow randomly - this is because the game has compatibility issues with dual core CPUs. So not only is there no performance boost with dual core CPUs, there are actually issues which require the following steps to be resolved:
Setting Processor Affinity Manually: Launch Need for Speed: Most Wanted, and as soon as the game appears to begin loading, or after it has loaded up, press CTRL+ALT+DEL to bring up the Task Manager. Under the Processes tab, find the Speed.exe process, right-click on it, select 'Set Affinity' and in the box which appears, untick 'CPU 1'.
Setting Processor Affinity Automatically: If you want to force the game to only use one of your CPUs automatically each and every time you run the game, see the instructions here for using the Imagecfg utility, a small Windows NT file you can safely copy into your \Windows\System32 directory and then use to permanently set the affinity for particular programs. I recommend you backup your speed.exe file before applying this tweak, as the only way to undo it is to restore the original .exe file. Furthermore, note that you may have to redo this tweak each time you patch the game, since the executable will change.
Both these methods basically force the system to effectively see, and use, only one of the CPU cores on your dual core CPU to run the game, thereby temporarily resolving this problem.