It's a well-made game with a great concept, congratulations!
In my opinion, I passed through all the levels with relative ease; I don't think there was anything which would be truly challenging or which needs more than two or three tries. You can slow down any number of moving objects indefinitely with a barrage of spells with fast fire-rate and turn very slow hovering in the air on/off as many times as you like, so you always have plenty of options to make the level easier or to fix your mistakes even mid-air.
Perhaps there should be areas which don't allow using your abilities (or moving objects unaffected by them) or some limits on your abilities. Not to make the game very difficult, just to up the challenge a bit.
It would also be interesting to see a greater variety of puzzles. What you have currently is fine, it's just that similar "tricks" and mechanics are repeated very often, well after you proved you know how to do them ("open door, slow it down, run through", "slowing human on a button" etc.). Syncing the blades in the last level was a good idea, perhaps you could do something similar for example to choose the order in which buttons are pressed by slowly falling weights (so you could run through a series of doors), propel yourself with a conveyor belt (which you have to walk across while it's slow) or something else which makes you use your abilities in a new, previously unseen way.
But those are just tips for improvement. The game already is very good as it is. :)