Well I think there's a few things going on here. I can tell you from experience that getting higher load WP sites working well takes a LOT of system memory, esp

Well I think there's a few things going on here. I can tell you from experience that getting higher load WP sites working well takes a LOT of system memory, especially it seems, when you enable WPMU. It is hard for me to pinpoint one culprit for this, but other high-load PHP applications I've dealt with run just fine in standard apache (no php-fpm+MPM module X junk). From what I've seen essentially Wordpress is just super heavy and tie that in with a more memory hungry MPM+PHP it quickly leads to Apache lockups. I don't think this is a problem with Apache at all, nor even a PHP problem but simply needing to have lighter-weight PHP applications, otherwise you are going to have to start resorting to tricks to get some good RPS counts out of your configuration. My end of story: Wordpress is awful to scale in Apache and it's not an Apache problem.