One of the best I've played so far!! Great level design and difficulty curve (loved the level where you got the key and then the whole level changed and level 12 rush to safety under the lowering caves)
Few things that got me really frustrated and would have probably quit if I weren't a perfectionist like I am :D
1) The goal area trigger/collider could be a bit bigger, since at least when trying to get 3 starts you have to speed up in every possible area and speeding up to the goal after a perfect run and then only to hit a small wall when you think the goal should have already been there, was very annoying.
2) Although I loved your level design and the look of the levels, some of the walls were really hard to see which part kills you and which part lets you go "through" it. Especially level 12 was annoying with the spikes at the end almost not visible and the red tube thingy which was cool looking, was set that way that in end half of the map you thought that it was just on the background and then it hit you...
3) This is more of a personal thing, but I didn't like the time freeze power ups, I only used them in level 12, where you pretty much have to use it, otherwise I just skipped them completely :D So hard to control when it is slowed. Also you should put a music or something on the power up, so that you know when it is about to end. I had this happen to me few times that I was vigorously pressing space and then it got back to normal time and I hit the walls :D
But overall I would say that it is an amazing game, maybe spice up the end scene a bit more, to make us feel more rewarded at the end, since I took maybe like 30 tries to make it on the last level :D