I mean hard to implement efficiently; that's still very possibly true on non-tagged hardware without custom microcode. But maybe not so we'd really notice outside of micro-benchmarks and extreme situations.

Since when was tagging on stock hardware was introduced by Common Lisp?

Don't tell me other Lisp implementations were faster than CL, because of not using tagging.

Today LispWorks, Clozure CL, SBCL, ... on my laptop run a thousand times faster than any other Lisp implementation ever did on a physical Lisp Machine. Actually they also run code much faster than Python, Ruby and most other popular scripting languages - often by a wide marging.

I think you repeat myths from 1984 where some implementors were claiming problems on small 1984 CPUs. They detected early that there were no problems compared to the Lisp Machine. You know one of the reasons the Lisp Machine was no longer popular for Lisp users from 1988 onwards? Stock hardware was a fast for many cases and much cheaper. Implementations like Lucid CL, Allegro CL and LispWorks were on workstations very competitive.

In reality, today on 64bit Intel chips, it runs just great. Even on something like an 32bit ARM, Clozure CL runs fine.

For many cases, good native Common Lisp implementations will run faster than Clojure.