Interesting game.
Looking at your pay attention to, don't worry about optimizing your game before you have it in a playable/feature complete state. That is priority #1 of any piece of software. If you start running into issues, profile it. Godot has a great profiler! I ran into no issues performance wise. So I think you were fine there.
I think there is much more you could do with the power up situation. I think this is one of those instances where more levels to try the power ups is good, or allow the player to choose power ups earlier to test them out. We didn't really get to see them which is a shame. Some of the slimes were hecka aggressive, Like trying to get the egg was so hard even with the Honey item. I think on restart your power usage should regen. Also, try when youre creating your music to loop to try and have it loop seamlessly. In your DAW should be able to loop a section over and over again, you might have to cut it down or re-record till it sounds seamless.
I really loved the bread and the slime animations! I think your level design too worked well. I didn't have any sections I couldn't get to!
Overall good jorb!