In addition to ccl, I've tried to use sbcl 1.0.29 with threads enabled, again on OSX 10.6. It displayed a similar slowdown with simultaneous connections. But with 100 connections, sbcl crashed and went into the ldb debugger: > fatal error encountered in SBCL pid 38459(tid 75514880): fault in heap page 12293 not marked as write-protected boxed_region.first_page: 0, boxed_region.last_page -1 Error opening /dev/tty: Device not configured Welcome to LDB, a low-level debugger for the Lisp runtime environment. ldb>