Having a lower base resolution with some upscaling would probably give a good boost to the performance.
As for the slowdown in the later level I didn't have the impression there were more objects on the screen than before (and there were certainly more at the times I shot an asteroid and all its fragments blow all over the screen).
A continuously worsening performance could be a sign that you spawn more and more objects (bullets, asteroids) that don't get cleaned up properly once they served there purpose. So either re-use them or make sure they get cleaned up properly, so they don't add up and drain the performance away...