Several people have kindly ported my ray tracer from this page:

  http://www.ffconsultancy.com/free/ray_tracer/languages.html

to Lisp. Some of them are reporting competitive performance. However, when I
try to run their programs with either CMUCL or SBCL they are two orders of
magnitude slower. Given the number of people claiming similarly good
performance, I'd like to know what the possible cause of the relative
slowdown on my computer is?

My system is an unladen 900MHz Athlon T-bird with 768Mb RAM running Debian
testing with SBCL 0.8.16 and CMUCL "19b-release-20050628-3 + minimal debian
patches". Other people have both slower and faster CPUs and more and less
RAM but all are consistently much faster than mine.

I believe SBCL always compiles to native code and I am asking CMUCL to
compile to native code with:

(compile-file "ray4.lisp")
(load "ray4.x86f")
(time (main 6.0 "image.pgm" 256.0 4.0))

Any ideas?