I think you might be thinking about Kazu Yamamoto and Mighttpd2? (Written in Haskell, incidentally.) I think this was the web server that beat out nginx on a largish number of cores (42?). For the life of me I can't find the original paper with the comparison graph, otherwise I'd link to it...

Anyway, a lot of the performance work which went into mighttpd2 is automatically shared by all Haskell applications. (Most of it went to optimizing the GHC VM I/O Manager which handles I/O in the Haskell runtime. It's sort of like libev/libuv only at the VM level instead of as a library.)