Start the light tracking static image rendering, the computer stuck CPU occupied 100%

hello all

Start the light tracking static image rendering  ,The computer will become very card graphics window can not rotate. the computer stuck CPU occupied 100% What is the reason? My graphics card is NVIDIA GTX750Ti.Given the material when the material is also very Dayton，

When rendering can be used without using the CPU only use the GPU, because I found that sometimes use the CPU to cause the computer to crash.

2.I do not want the light trace to open and cause the computer Caton model to not rotate.
keyshot rendering will not cause the computer card, NX start ray tracking window after feeling nx fluency will be worse, the software becomes very card, can not smooth operation nx.