Lisp machines weren't slow; the original CADR of the late 70s ran at around 1 MIPS on 32 bit data with up to 8 MB of RAM, making it about as fast as the VAX 780. The VAX was a large minicomputer released in 1977 and one of the fastest machines, short of a high-end mainframe, at the time. A Lisp machine also cost about as much as a VAX (but for a single user).

The problem was maybe, aside from a $50,000 PC being hard to sell, that even on such generous hardware with specialized support, Lisp, particularly with the more naive compilation techniques of the 70s and early 80s, and after adding a fairly sophisticated operating environment, was still a rather hefty language.