And when did Lisp become blindingly fast?

Well, I'd say that Lisp, being merely a language, and thus somewhat of an abstract thing, is neither slow nor fast. For individual implementations of Lisp, one might give the rather tautalogical but nonetheless true answer that they became fast when implementers figured out how to make them fast (which incidentally doesn't relieve the programmer of his responsibility to help out, but that's another story entirely). Modern Lisp implementers have done a pretty good job of holding up their end of the bargain; good examples are CMUCL and ACL. (I myself am a user of Lispworks which isn't too shabby either.)