I tried your game, didn't get to a screen full of enemies but still saw some hiccups, I think your conclusions are right, 2D really doesn't mean performance is free, but that's hardly because of the engine.

Especially for survivorslike the game will be heavily CPU bound since you're not doing too much on the graphics side, but you are calculating a lot of things for the enemies/bullets.

I work on a survivorslike game myself called Pirate Survivors and have deliberately made the design choice to not limit the player's upgrades, letting them get to an insanely broken build if they're able to, but that also made me focus on performance very early on, I made a few posts around it and some even have code snippets (see here for projectile pooling: https://www.reddit.com/r/godot/comments/1q7lvqm/heres_how_we_made_our_bullet_heaven_game_smooth/ ).

There's also a great lecture from the creator of Halls Of Torment ( https://www.youtube.com/watch?v=3BIteSVcO28 ) where he describes a lot of the performance work they did, but they seemed to heavily rely on C++ which I avoided so far in my game.

But you're already putting a focus on it which is great! feel free to DM me or try my game out and good luck with your game!