I'm just going by what his writer said.
He blatently said his reflexes and reaction speed is why he can keep up with Flash for the first 2-3 seconds.
"He's not a speedster like the Flash, but he's faster where it counts."
"No one's ever tested Slades speed before, but for the first few seconds, he can hang." -When asked about Slades speed compaired to Flash.
A few other quotes that I can't remember the exact wording, but he definatly insinuates that it's not all about calculations.