I had this problem too, after upgrading my comp recently, but I dont think this has anything to do with multiple cores.
your crippling speed boost is being caused either by
A: Your CPU clock is too high,
B: your processor was built on a more efficient (read: faster) architecture, or
C: both of the above.
for me the problem was C, for those of you who know what it means, my processor is an AMD Phenom 9950BE.
There is an aplication that runs only with AMD CPU's called AMD OVERDRIVE, which is an overclocking program that you can also use to throttle down your speed, without even having to turn off your computer.
The program is very user friendly, just look for the HT ref. clock and pull the slider all the way to the left, then, find the HT Multiplier setting and pull that all the way to the left as well, then hit apply or OK.
If you have an intel CPU, all is not lost, just tell a tech-savvy (read: nerd) friend that your CPU is too fast and ask them to show you how to underclock it using the bios.
NOTE: AMD OVERDRIVE WILL NOT WORK ON INTEL CPU'S. IN ADDITION, THERE IS NO GUARENTEE THAT YOUR MOTHERBOARD WILL HAVE OVERCLOCKING OPTIONS.
Good luck, Happy playing.