Joomla has just been completely rebuilt... so not sure about the old code philosophy you cite. Not sure how old the current WP code is, but I'm pretty sure last time I looked, it's using HTML 5 elements.
I haven't come across a CMS yet that is feature rich that doesn't have some server drag due to queries. You can't have the best of both worlds... you can't have every feature and option known to man, and have super fast on the lowest grade server spec.
Its usually lots of features equals decent server hosting. Limited features, cheap hosting.
Xenforo is using OOP and the Zend framework, and also now having to implement caching strategies and performance factors, because as they increase functionality, it will increase queries, thus server load to some degree. Keeping it as minimalistic as possible is the aim IMHO... which I believe people like XF are excelling at.
Joomla 1.7 isn't too bad for load, compared to its earlier, outdated 1.5 code that could crash a server with a good spike.