http://www.joelonsoftware.com/items/2006/09/01.html

Actually, despite the fact that I love Ruby a lot, I'm inclined to
partially agree with him on this. Presently, our company does have
some Rails-based web applications deployed but they're predominantly
applications geared for use by only a few people (internal client use
only); we've not yet tried to deploy a real public-facing web
application based on Rails. For that, it works really well. We're
taking a wait and see attitude before we attempt to use Rails for any
high load applications; my own experiences attempting to optimize
plain Ruby code for performance have been simultaneously frustrating
and rewarding. I doubt I could do the same with a Rails app. So for
now we're gonna stick with PHP for our public facing web applications,
even if it is even worse for i18n/l10n/m17n applications than Ruby
is...