After an acident in the research of parallel dimensions, a portal was opened and one of the researchers fell inside. He was successfully rescued, however the rescued scientist wasn't like before. While he looked the same from the outside, his body was nearly indestructible, his strength was immense, he could resist temperatures much higher than the sun's core and his brain and body could function and react normally at the speed of mach 50, allowing him to move faster than the human eye can see (these are the effects of becoming bound to the laws of another dimension, and having his body changed to fit said dimension, an event which wasn't reversed once he returned). In this question, I'm interested on his perception, especially since, in our own case, we have the feeling that things are happening in slow motion in some cases of flight or fight situation, despite the rate at which time passes not having changed.
With a brain and eyes that react to the world at mach 50, to what extent would the world look like it's in a slow motion, based on the perception speed of the normal eye from an average human? I'm having problems estimating how exactly would he see, and that's important to determine how he'll react to the world around him.
Note/edit: the character is not trapped in a different time rate, but "merely" has a body capable of processing and reacting to information much faster than a human normally could, and ages at a normal human rate despite having a different body. Think of it as an extreme version of listening to a sped up version of a song many times and then suddenly listening to the normal one again, extended to his senses and with a muscular system capable of accompanying such thought speed.