Flash time traveling question in Fast Forward Episode

I have never quite understood something here.

In this episode, I assume that the combo of his speed and the missle explosion make him go faster than light to leap 10 years in the future.no problem there.

But why is it that just because he time traveled into the future he is unable to use his power? what's up with that?

he clearly had it, Mcghee said something about metabolism being unstable. he was able to do to it for a few seconds when the maching Pike used jolted him, so he certainly has it, and McGhee made something that said it would last 10 minutes.
But when he goes back in timethe same way he does not lose his power.
But why on Earth would the travel itself casue that? that never made any sense.
it would have been so much easier for him to accomplish what he needed if that weren't the case.

also. does anyone know how fast the missle was going?