Houdini 18.0566
RTX2080 in eGPU enclosure

I've set my solver to Opencl and the gpu is set to the 2080RTX in preferences>misc. But when I play my CPU is throttled while my GPU is only 20-30% utilized. Is this expected?

That looks correct, because more than likely you are not following all the rules to maximize GPU acceleration. The default tool generates a network that can only be partially accelerated, and you are getting that. Not all nodes can be GPU accelerated and when that node needs to be evaluated, the CPU is used in that case.