While not live, I've run some load tests with a relatively stock XF installation and simulated high traffic... The results are pretty good...'
The Good
Seems to be 4-5x less server resources needed vs. vBulletin as well as a substantial number of fewer queries. For query count (as well as the EXACT query being run), check out
http://xenfans.com/help/ for whatever reason they have debug enabled. 2 queries for a minimal page is really good in my opinion (but if you browse around, you can see the query count as well as exact queries used). Sessions are stored in your cache (for example memcache), templates are stored as PHP files (bonus because it's a lot of PHP code that doesn't need to be run through eval(), and instead can be pre-compiled by your opcode cacher).
If you do a lot of custom stuff, most everything I wish vBulletin did internally, XenForo seems to do or at least have the ability to do (even if it's not currently done). For example, I just got done coding something that I want to cache, but not in the registry/datastore... something not important, so losing it if the server reboots is no biggie. Since XF uses the Zend framework for all that stuff, it's as simple as this:
PHP:
if ($cacheObject = XenForo_Application::get('cache'))
{
$myThing = $cacheObject->load($cache_key, true);
}
and saving is the same deal...
PHP:
$cacheObject->save($myThing, $cache_key, array(), 900); // 15 minute cache
Now obviously you could go straight to memcache, but it's nice to be able to route through the native framework... that way if you change your cache backend, you don't need to change code everywhere.
There are no issues running it on multiple web servers through load balancers (other than some things like avatars being stored in the file system... see below).
The "I wish it was supported"...
There is currently no master/slave ability with XenForo... one DB server, and that's it. You *can* specify a database adapter namespace in your config, which more or less would allow you to make your own DB class that extends the existing one to do it. I started working on one... got about half done with it, and then decided my time would be better spent building things that I know will never come to the XF core. Then when all THAT is done, I can go back to it.
http://xenforo.com/community/threads/suggestion-mysql-read-slave-write-master.14492/
The addon/hook architecture is fantastic... unfortunately it's not as efficient as it can be, and overall speed of pages start to noticeably degrade the more custom stuff you have (even on pages that don't have any of the custom stuff on them). Thankfully, it's not a terribly difficult fix... Hopefully something along these lines gets incorporated into XF:
http://xenforo.com/community/thread...-type-into-single-php-file.22301/#post-281417
Storage of avatars in the file system... Currently they are always stored in the file system, so it makes multi-web server setups slightly more annoying. But with the hook architecture, you can more or less override the avatar model class and simply do it DB based (or however you want)... just that you have to write the code.
The bad
Sometimes I want to stab the editor... WYSIWYG mode loses all tabs, so when you post code and such, it's really, really annoying.