Time manipulation is a really interesting power. I mentioned in another comment, that if you moved in normal time, whilst the rest of the universe was moving at a slower relative time, then you would also be aging faster relative to the rest of the universe! That would mean each time you used the power to slow down the universe, you would be getting older compared to the twin that was in the "normal" flow of time?
I think time travel is really interesting as well, although, I would only do it to view things and inform my curiousity! Like you said, it could be that trying to change something in the past might make a worse problem than the one you tried to fix!