I've actually figured out that it's caused by having the in game vsync turned on. When disabled it's fine, but I'm not sure why it's causing an issue as I'm also using freesync.
Do you also have the issue with D4 where the in game menus are laggy when opened? For me the GPU hits 100% usage and the fps drops. Once I close the menus/inventory it's fine.
Yes it's wasteful. Do what I did, save yourself the money and start making your own water, it's extremely easy and costs next to nothing. Check out https://www.tinkercoffee.com/blog/2017/11/9/thats-some-high-quality-h20