My dad's W510 had been acting painstakingly slow for some time. We found out that this was because of the 90W ultra-slim adapter (the W510 thinks it needs more power and slows down the processor and gpu). A quick fix is to pull out the power plug and to put it back in, this resets the power manager software.
However, this gets frustrating when you have to do it every couple of minutes. Someone at the lenovo forums suggested a more permanent fix: shorting some pins under the keyboard (read the forums for more details). You should only do this on your own risk, and you need some technical skills, but in our case it does work!