Thanks for the really specific feedback.
I actually didn't realize the music system was causing the lag because on Windows the game consistently runs at over 140 FPS. I assumed the performance drops were related to chunk loading when entering and leaving the mine, especially since many 3D Godot games struggle in browsers.
After reading your comment, I investigated the issue and realized that the problem was indeed the music transition being triggered every frame. Once I fixed that bug, the browser version now stays above 60 FPS, and the lag is gone.
As for the AI comment, no, I didn't just prompt an AI and ship the game. I spent a lot of time building it, testing it, and fixing bugs before releasing it. I totally get why you'd be frustrated by the performance, but it honestly sucks to have the whole project written off as "just AI prompted" when I put a lot of work into it.
The lag is fixed now, so I hope you'll give it another shot. And if you run into anything else, I'd appreciate the feedback.