Choppy playback with high end system

Not sure if I'm expecting too much but I'm having perpetual issues with playback in Premiere Pro, whenever I have any kind of transition or effects on 4K footage... arithmetic, directional blur, zoom transitions etc the playback is unusable.  It freezes for 1-2 seconds at a time making the editing process pretty impossible.

My system is a 8700K under a X62 AIO at 4.9GHz, 32GB 3000MHz Trident-Z RAM, GeForce 1080Ti, Samsung NVMe M.2 SSD, I have GPU CUDA accelerated Mercury playback enabled in the project settings, but even playing back at 1/2 resolution is way too choppy to be any use.

I actually don't think with a system like that I should be suffering choppy playback at 1/2 resolution with only a couple of effects on 4K footage, so I'm inclined to believe I need to optimise some settings perhaps? Does anyone have any pointers?

Cheers! Neil.