Hi Attila,
About the encoding -- don't worry about ditching trivial-utf-8, if there is a faster and more general alternative that's cool. I don't see any functionality for writing directly to a stream in it though, but maybe I didn't look hard enough. Perform some benchmarks to prove that babel actually speeds things up before building a patch.
I didn't know iolib. It looks cool -- but I'm not okay with making postmodern posix-only. If you can find a way to make the socket library 'pluggable', that would be cool... but then we're writing a wrapper around socket wrapper libraries, which is a bit screwy. Have you looked deeply in the problems you're having with usocket? Might they be fixed by patching usocket?
Cheers, Marijn