Dan Barlow comments on his experiences using cmucl (well, sbcl) as a web server (including cliki):

For the record, the site that served you this page is running on the pretty similar SBCL (you wouldn't guess) using Araneida, and doesn't even use MP, instead hooking into the same event loop as SBCL uses for IO at the top-level. It does have Apache in front of the CL web server to act as a proxy, which tends to avoid most of the "slow/malicious client" problems.