Ever try to overclock your CPU ?
Try to OC your CPU by 10%, then record the diferent (FPS)
Make your CPU normal, then OC your GPU by 5%, then record the diferent (FPS)
Normally, if :
CPU (OCed) FPS > GPU (OCed) FPS, then you got bottleneck CPU.
CPU (OCed) FPS < GPU (OCed) FPS, then you...