I think "serius" is better than "tardius" here. "Tardius" fits best with the sense of "tardy" in English or "late for an appointment" or "delayed" or "behind time",—unless you want it to mean that, of course.
"Serius quam putas" or "Serius est quam putas" (the "est" for "is" is optional).
To me the bit "plus ut putas" suggests "the more that you think". So "The more that you think, the later it gets. So stop thinking and act!" That's nice, but maybe not the original thought.
I'm writing in Latin hoping for correction, and not because I'm confident in how I express myself. Latinè scribo ut ab omnibus corrigar, non quod confidenter me exprimam.