Over heating & machine lags due to the same reason.
Disconnect the battery, Unscrewed & removed the heatsink & fans, cleaned the surfaces of the CPU & GPU & also the copper plating which is on the heatsink with 99% isoprophyl alcohol thoroughly till both the surfaces became shiny, reapplied arctic silver thermal compound on both the surfaces also on the heatsink & fixed them back.
Im an apple certified mac technician but work as a freelancer. I fix all mac computers hardware/software issues. My advice would be, do not attempt to open any mac computee r without prior R&D & knowledge of the machine. You might end up ruining tiniest of the tiniest part which might cost a bomb on your pocket.