FWIW, speaking from memory, if QPX, Google's (previously ITA's) low airfare search engine, is any indication, then the lack of any real mention is not surprisin

FWIW, speaking from memory, if QPX, Google's (previously ITA's) low airfare search engine, is any indication, then the lack of any real mention is not surprising because QPX did not make use of parallelization or concurrency. The way it was written made that move very difficult. From what I recall, there was interest in parallelizing some bits of the computation, but I don't recall it ever really going anywhere. (QRES, ITA's reservation system, which was also written in Common Lisp, may have made use of concurrency or parallelization in some way, but my knowledge of that system is limited.) N.b., QPX did not quite follow all of these recommended practices during my tenure (e.g., ubiquitous SETFing of object slots).