rewriting in a lower-level language won't do too much for NC, because it's mostly slow due to inefficient IO organization - things like mountains of XHRs, inefficient fetching, db querying etc. - None of that will be implicitly fixed by a rewrite in any language and can be fixed in the PHP stack as well. I think one of the reasons that helped OC/NC get off the ground was precisely that the sysadmins running it can often do a little PHP, which is just enough to get it customized for the client. Raising the bar for contribution by using lower level languages might not be a desirable change of direction in that case.