Why do TPTP Performance plots look like this?
I can speculate/hypothesize.  Each solver has many heuristics and proof tactics.  Imagine that corresponding to each heuristic is a class of instances that are easy to solve if you use that heuristic, but hard if you don't (e.g., if you don't have a heuristic for it and you fall back to the generic methods, maybe you're effectively exploring the entire exponentially large space of possible solutions to try to find one that works).  An example would be 2SAT; if you use the right algorithm, it can be solved in linear time, but if you blindly search, that will take exponential time.

So, in this model, if an instance is handled by one of the prover's heuristics, it is solved very rapidly; otherwise you fall back to some generic method that may take exponential time.  This would lead to a kind of "very fast or very slow" dichotomy.  And the set of instances that a prover is fast on may be different for each prover, because prover uses a different set of heuristics.  In other words, this hypothesis would explain the observed behavior.

I have no data and no hard evidence, so this is merely speculation.  It could be entirely wrong.