They all have badly designed code. They are old. They need to start from scratch with a proper design. I see the listed software like SMF, phpBB and vB. All dated and hacked and bloated. To the end user they are ok I guess.
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Joomla 1.7 isn't too bad for load, compared to its earlier, outdated 1.5 code that could crash a server with a good spike.
Wordpress is the bomb but is Kier really going to provide a CMS? If so then I'll switch over because my Xenforo forum is faster than my main Wordpress site even when I use different themes. Will Xenforo also come out with blogs?
I've never been into "Joomla", but I know a lot swore by it. I used to be a big WordPress fan, but I dislike greatly the changes made with it since. Sure, you can install a custom theme with it. But what on earth made them use that new default theme called Twenty Eleven?
I used to own VIVVO but sold my license soon after to another person on vBulletin at the time. It's looks even more like a classy CMS today, I still like it and rate it as being the best looking of the bunch using the stock theme design. But, I never could understand why it runs so slooooowwwww. Even running it locally on WampServer to avoid my shared hosting test, it ran like a snail.
That's why I sold my license soon after buying it, the slow running speed was driving me nuts.