People called the post "epic" and canonical in the comments. Does it apply to XF, though?
Which Apache MPM do you use? Have you tried Event? It looks like just a couple of clicks to upgrade to it if you have a cPanel/WHM VPS.
And should I try it? That would make my cPanel stack into Apache2.4/Event/FastCGI/PHP5.5. My old VPS has Apache2.2/PreFork/SuPHP/PHP5.3, and I'm pretty lucky to have two VPSs at the moment to play with, so I can experiment before migrating my site. The apps we run, over 5 cPanel accounts include XF, Mediawiki and WordPress.