I see other people are also having the black screen when alt-tabbing. Not a big deal, since you don't really need the game to continue doing things, but it is a whole 1-2 seconds of my life I'll never get back, so woohoo.
I decided to run a Simulacrum to see how it changed. Ran Wave 1 before I realized that I needed to change the setting at the login screen, so Wave 1 was a nice baseline of DX11. It was choppy, frame rate was all over the place, felt like it had all league.
Relogged, switched to Vulken, lost a portal, oh noooooo, got back to the Simulacrum. The frame rate is still not constant, bouncing around between 60 and 30. However, throughout the entire Simulacrum, I only had one moment where it felt like the game lagged, and it was just for a moment. 19 waves of, yes, admittedly inconsistent framerate, but the game didn't stutter. It was never an unplayable dumpster fire. Can it be improved? Sure. Is it an improvement already? Yes, I think so.