Ah.. I think I see the miscommunication here. By slow time, I mean he slows [i]his[/i] time. I'm just using it as shorthand for his perception of time: I'm not shortening it to PoT, cause that could mean something else entirely XD

And it's only outright invulnerability [i]to a limit[/i]. When that limit breaks, he is pretty much immediately dead based on the speed that he could be going at. Now, the size of the limit varies on how much energy he puts into it. How much energy he puts into it determines how long he can keep it up. For stuff thats not particularly fast, he could keep it up for longer. For stuff that is incredibly fast, it would be for a fraction of a fraction of a second. And as I've said, it freaking [i]hurts[/i] to use it, and that pain is just going to get worse when he wants to do stupidly fast things. It'd be like he's pulled a muscle without the actual damage: it makes it borderline unusable.