I used to think about this way too much. The answer is the Speed Force, the power that various Flashes tap into. So he can move around like a normal person fine. When he wants to go fast, he taps into the Speed Force. He’s not like Quicksilver, who is just fast all the time and has to actually physically slow his body to have a normal conversation, etc. So, I’m sure it’s frustrating because he knows he can just use the Speed Force to do things, but he’s not necessarily perceiving time differently on a regular basis.