Actually it sorta makes sense that your CPU speed is dropping..
The Intel i7-720QM is a quad core processor that turbo boosts to a maximum of 2.6 GHz according to the number of cores that are being utilized.. Say your app only uses one core.. the processor will switch off 3 cores and turbo 1 core to 2.8 GHz... it sounds like your rendering program uses all 4 cores; consequently, the speed drops to the rated nominal 1.6 GHz for 4 cores... This is done by design i think....
tldr; the more cores the program uses, the lower the speed will be... Reference : Intel® Core
EDIT: Beaten to the punch =P