Special purpose CPUs ran faster than general purpose. However they had upgrade cycles of 3-5 years compared 1/2 to 1 year for commodity chips. The commodity chip almost always caught up in the meantime at a lower cost. My research group bought array processors, fine grained processor like MassPar and Thinking Machines, min-super computers like Convex, and this catch-up happened every time. LISP firmware on general CPUs caught up with custom hardware like Symbolics too.
Very large customer bases like Nvidia can have annual design releases and keep up.
> The commodity chip almost always caught up in the meantime at a lower cost.
This dynamic is dead now, thanks to the slowing down of Moore's Law. We're even seeing a resurgence of special-purpose hardwired accelerators in CPU's, because "dark silicon" (i.e. the practical death of Dennard scaling) opens up a lot of opportunity for hardware blocks that are only powered up rarely in a typical workload. That's not too different from what the Lisp machines did.
Very large customer bases like Nvidia can have annual design releases and keep up.