I ran into some serious performance issues - like other users I frequently got "WebGL context lost, please reload the page" messages after the game loaded and before it started. Every once in a while it does start, but when it did there was lag and the music was extremely choppy.
There might be an issue that's being pointed to by some odd errors showing up in the browser's Developer Tools - Console window (if you play the game from NG, press Ctrl+Shift+I or pick it from your browser's menu and go to the Console tab to see them). It makes me think that an autoload script might be acting up, but you might be able to make more sense of it than I can since I don't have the Godot source code. If you want me to take a look at it when I get a chance then msg me.
Other tricks I've found in Godot that might help with performance if you're not already doing it this way are 1) for most of my big scenes (like game levels or main menu or stuff) I'll make an autoload that holds the entire scene as a variable like
levels_autoload.gd:
var level1 = preload("res://levels/level1.tscn")
etc...
and then in other scripts instead of using get_tree().change_scene() you would use get_tree().change_scene_to(LevelsAutoload.level1) and it should noticeably reduce if not eliminate the stuttering and lag when you switch scenes.
2) For audio, maybe try compressing the music files in Audacity. If they're in stereo then mix them down to mono (Ctrl-A to select all, then Tracks -> Mix -> Stereo down to Mono) and then do File -> Export -> as OGG with the Quality slider set to 0.
3) The other thing you could do with audio is significantly more complex, but if you're up for it I described the process in a post at https://3p0ch.newgrounds.com/news/post/1148893
Hopefully it can get running more smoothly because now it's kinda barely playable for me.