I read this story probably in the early-to-mid 1960s. I believe the Flash (Barry Allen) either went to another planet or (more probably) a parallel dimension (possibly by means of his Cosmic Treadmill breaking "the dimension barrier"?).
(I first came across the dimension barrier in the answer to What is Barry Allen's aka The Flash's top speed?.)
In this place, the Flash is surprised to find super speed is the norm. In this world, a building is erected in super fast time (to which the comment from someone is the surprise that it took so long), but also, there is a super fast tortoise. The tortoise is not the Terrific Whatzit, and the artwork was of the Silver Age.
