Lisp machines weren't slow. The initial CADR machines around 1980 could run simple integer and maths code around as fast as the VAX/780, a very fast minicomput

Lisp machines weren't slow. The initial CADR machines around 1980 could run simple integer and maths code around as fast as the VAX/780, a very fast minicomputer costing ~$50,000 (processor only!) that was released around the same time. And yet Lisp machines were slow. I chalk that up to a proto-Common Lisp system, with 1970s compilation techniques, being just too much. Millions of lines of code of a fully dynamic software environment with a GUI debugger that let you step into any part of the system including the OS and lowest runtime layers and edit it live -- crammed into a couple megabytes and a couple MIPS. Of course it was slow.