Fun game, nice! There are some lags in the beginning, like on first hit. Maybe it is because of shaders? Can you try precompiling them on app start? I had similar problems in my game, where for example the first emitted particles caused a lag like these.
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Yes, they must be in view. I had one material off screen and when rotating the camera in it's direction, it lagged the first time. I think that with dynamic lights, there are multiple shaders for the same material. I am not sure about this, because it was couple of months when I first looked at precompiling shaders on Godot.