Bootcamp lacks driver support for the iMac Pro GPUs - Vega 56/64. Bootcamp seems to be using some kind of generic GPU driver with very poor performance. Most games are either performing poorly, or are downright unplayable (very low fps, crashes, instabilities etc.). Please update the drivers. It's a shame that a $5000+ computer with a high-end GPU, lacks a properly working GPU driver.
Thanks for listening,