I tried Proton 11, 9,5, and 10-GE. But the issue doesn't have anything to do with Proton. I realized a little later than all my games on PC weren't running well. Seems to be because I switch over to Wayland recently. When I changed back to x11 everything started running smoothly again including your game.
I made it to the boss fight but haven't beaten him yet. I found Project Goblin through r/Godot. I'm also working on a lowpoly action game (more soulslike than DD though) and its really motivational to see that one person can put together a great game like this.
If you do decide to make an native linux version and need someone to test it feel free to reach out.