Open it out and brush out all dust, especially around heatsink and fan. This should help any laptop run cooler. Computers collect dust over time and the dust causes high temperatures which causes GPU/CPU throttling which causes lag. You need to clean out your computer once in a while.
To check the temperatures you can use iStat Pro or Macs Fan Control (both free).
P.S. the fans always run, just at a base speed of 2000rpm so you don't hear it. They don't turn on or off.