I've had this slowdown problem for a while now with none of the fixes working. I couldn't disable speedstep in bios so I went looking for a throttling program. Downloaded Throttlestop and may have found a semi-fix for those who still are having problems.
I have the unlocked x9100 C2D which let me play with the multiplier in throttlestop. I set it to x6 multi while in game and.. yay, no more slowdown! It seems to be linked to cpu load (lower multiplier means lower cpu speed so EQ eats up more cpu time). Tested it by running at high cpu speed again, but this time with Prime95 maxing my cpu. No slowdown at 100% usage. I guess speedstep can't kick in if you have a high % usage.
For now I'm using
http://www.fossiltoys.com/cpuload.html to simulate enough usage to cancel out speedstep but not to cook my laptop. (now looking for a program to dynamically reach the speedstep cutoff instead of a static amount)
Hope this helps!