Recently I did a consulting job for a project of a kind of network backbone testing suites used by one of major telcom providers in Japan. The suite was mainly in CL, but my part was an independent, stand-alone tool and I used Scheme (Gauche). It was a distributed, multithreaded app that opened raw sockets and sent/received fair amount of ICMP packets. The main app was just around 1000 LOC in Scheme, though it used quite a few libraries came with Gauche.

Speed didn't become an issue even Gauche wasn't a native compiler (it compiles to VM), for the network driver was always the bottleneck. Yet I did care not to cons in the inner loop.