We consider first-passage percolation on the class of "high-dimensional" graphs that can be written as an iterated Cartesian product G G . . . G of some base graph G as the number of factors tends to infinity. We propose a natural asymptotic lower bound on the first-passage time between (v, v, . . . , v) and (w, w, . . . , w) as n, the number of factors, tends to infinity, which we call the critical time t * G (v, w). Our main result characterizes when this lower bound is sharp as n → ∞. As a corollary, we are able to determine the limit of the socalled diagonal time-constant in Z n as n → ∞ for a large class of distributions of passage times.2010 Mathematics Subject Classification. Primary 60C05; secondary 60K35, 82B43.