Well it will depend on the impulse loader but using something like Redwirez is just the same as convolution reverb.
I've found the amp sims I use use more CPU than loading Redwirez IRs, but it will depend on the convolution plugin ofcourse.
There is however, this;
GPU Impulse Reverb VST | NVidia/ATI GPUs used as DSP for convolution reverb calculation So you can load all your cab impulses or convolution reverbs in GPU instead of CPU