A word of explanation - about Lisp power.
What I mean is not that Symbolics machines are the fastest CPU at processing (they are complete clunkers by today's G-Hz CPU hardware standards). Rather that they are superb environments for the programmer to use to get their brains around new designs.
We can all nip over to borrow the nearest PC word processor with its cooking Intel/AMD multi-cores inside if we need CPU power. But what help is there out there for the designer/developer, the software engineer who knows what they want to do but not how to do it yet, who is not painting by numbers - i.e. is working something out for the first time.
One often reads how Lisp programmers cost a lot. Of course untrue, but the point often missed in the reasoning is that these people are often doing very difficult things, which generally carries cost burdens. Think code mechanics v original thinkers (at least in that direction).
In summary, by power, I mean power tools for thinkers and designers. Not high CPU MHz.
PS. On lisp execution speed see Ken Anderson's insights, and perhaps Paul Graham's.