Hello ari,
as I said in my other thread (link below): I have the feeling, as long you have just one GPU inside, dt seems to load the CPU once a task has been adressed to the GPU, no matter the GPU is fully loaded.
My case was GPU runs around 25% load and CPU on all (by that time 6) cores at 100% when pixelpipe recalculation was required, e.g. like your case, scrolling (the harder with “heavy-duty-modules” like denoise profiled or equalizer).
Then when you have 2xGPU things change. Suddenly the CPU-load dissapeared and things run way faster.
Meanwhile I further updated and have an i9 9900k. Still I can find scenarios, where things (whitebalance) can be snappier and both GPU not running on 100% yet. So opencl still leaves us space for improvement 
To me it looks like it is very closely linked to my thread here: (besides the topic about the beer )
I would be a happy tester, if needed 