Easy fix for the slowdown issue, switch to Graphics mode in the title screen settings menu. If you use Performance mode, the game will try to run at 120fps and you'll experience the slowdown. With GE-Proton8-15 and Graphics mode, I'm comfortably getting 60fps on medium-high advanced settings with no slowdown. I was averaging a bit over 2 hours of battery life. You can probably tinker with lower TDP and graphics settings to get more efficient battery life.

The only bug I've encountered otherwise is a frequent screen flicker on the skill upgrade menu screens.