I'm just saying, if your brain cannot keep up with superspeed, you're going to crash into everything. So their brain is processing just as fast as they're running. Hence, everything SHOULD seem slow-mo all the time. Unless it is a sort of thing they can turn off which I doubt
Alternatively, they process things at regular speed, and so need to carefully consider their route and steps before they can take off, limiting their reaction time but still maintaining a "normal" perception of the world
https://sundaecomics.com/2020/11/15/watchpeopledieinside/