Didn't have any performance issues on my end while playing.
Anyway, since its an early (and your first) prototype, you will probably mostly read things you already know yourself, however i'll just write down my thoughts:
A settings menu would be great. I always turn down sounds in game because i've set my system sounds too high, couldn't do that here. Also while at the topic of settings, you might want to check out the other fullscreen option in Godot, because it prevents the grey border around the window.
The explosions are way too slow, like 80s era game explosions while the enemy animations are pretty fluid.
Enemies obviously need more mechanics than just approaching and damaging on contact.
Weapons need projectiles, more visual feedback for both firing and hitting a target.
The sounds are actually okay, for instance jumping sounds like a mech jumping, but landing is stiff and quiet.
The rickrolling is a nice touch, didn't expect that.
Will the anime girl in the corner be animated later on? Like the doom guy reacting to damage, power ups etc?
Good job on making a stable demo though. Hope to see it evolve in the future!