Try to force Directx 9. I had the same issue with Shantae Half Genie Heroe and dx9 mode fixed it!
Really didn't see a way to do that.
PROTON_NO_D3D10=1 %command% in the launch command. Also (for me) "-dx9" also works
Tried both options, even with Proton 4.2.1, performance is just as bad. I suspect you may be onto something here, but not sure what exactly I could be doing wrong on my end.
Game will not launch on my deck unless you add "-nolauncher" before starting the game.
proton 4.2x1 2019-03PROTON_NO_D3D10=1x1 2019-03PROTON_NO_D3D10=1 %command%x1 2019-03
Dungeon Defenders - Windows Edition
nVidia GTX 960, driver 418.43
Linux Kernel 4.18.0-16-generic #17-Ubuntu SMP
Proton version: 3.16-8 Beta
[X] that I haven't found an existing compatibility report for this game.
[X] that I have checked whether there are updates for my system available.
The game runs really poorly, the Windows version. The Linux version runs really well, but the game doesn't have cross-platform play, and my friends play the Windows version.