I got it compiled on Linux, and yeah, I'm getting roughly the same experience. (Slightly higher Framerate in Linux, though in both scenarios my framerate drops as the game progresses.)
My Windows build is here, but mileage may vary. It seems to work ok on my other Windows boxes. ~~https://s3.amazonaws.com/rrbits/dodgem/debug.zip~~ (Edit: Use the release build below)
Now for feedback: You made good use of the theme. A top-down shmup is a natural fit, and the slowdown when you pick up items. (I'm guessing I should have dodged those too?)
That said, the lack of instructions made it a little hard to figure out was was going on beyond 'Oh, I need to shoot those and dodge stuff.' Also, I couldn't figure out how to respawn after death, so I'd just close and reopen the game.
Be all that as it may, I still played a number of times as it's oddly addicting.