Yes it is pretty lame and random how they are attempting to optimise it.
Can stand in HOM with 20 people jumping around me and the CPU usage is at 90-95% whilst the GPU is making a cuppa tea and having a rest with its feet up giving me around 25-35 FPS. Then when I move out to a clearer area with scenery, trees etc. the GPU powers up and wizzes away to 99% and gives me around 80 fps.
Crazy times lol