I totally agree with the restart key, I didn't think about it in time...
Your idea to add a downside to the slow time mechanic is interesting, I think if I implement something like that, it would be together with a score system so that if you don't use it you get higher points but if you use it, you can still enjoy the game. A way to reward players who want a challenge without alienating the ones who want to relax.
Thanks a lot for your feedback!