Idle clock speed for Nvidia GPU.

[admin89]

It's possible that your laptop is overheating . Keep in mind that dedicated GPU has its own power consumption . Integrated Intel HD 4000 power consumption is low since it's inside CPU chip
Most multi-media laptop can't handle the heat when both CPU and dedicated GPU are used at full load . That's why gaming laptop exists
i5 3210M + Intel HD 4000 = 35W
i5 3210M + GT 630M = 30~35 + 35 = 65~70W !
Higher power comsumption = Higher temperature , need extra cooling solution