The speed and freedom of the movement is just right, although any level with verticality will render enemy encounters trivial, I'd consider adding a timer for levels, I'd also suggest unlocking levels into batches of 5 or 3 and requiring x number of things (be it stars or whatnot) to unlock the next batch so that players who are stuck in one level can just pick another one and are less likely to rage-quit.
Also the guns are quite underwhelming and definitely need to be made more unique from one another to incentivise weapon switching like in Doom