I've noticed a few references to speed issues both before and after the recent software upgrade and I took a look into this today. I did find a few problems that could potentially cause the system to slow down under load and have fixed them. I'm not sure as yet what caused them, but I'll be keeping a closer eye on things in the future. However, I would like to explain why the server is slower some times than others.
For starters, perhaps I should mention that the server that hosts the forums is a dual Opteron with 2 gigs of RAM -- pretty much the top-of-line when it was initially installed early this year. No expense was spared here! The only other service on the machine is the on-line store which (unfortunately) gets far less traffic than the forums.
The most common cause for "server slowness" has nothing at all to do with the server and is caused by traffic conjestion. The datacenter where the server is located is fairly well connected and the delays are typically introduced within the user's ISP. I have both cable modem and DSL at home, and I regularly notice periods where one will be slow and the other will work just fine. Not much you can do here but cope or get a new ISP.
The second most common cause is server backups. There are a number of administrative jobs that run between midnight and 4AM EST that do load the machine down rather heavily. I'm working to push them back a bit (4AM - 7AM EST is the period of lowest load) and have them run with a lower priority.
The third, and most questionable, is the vBulletin software itself. I have noticed that posts to particularly long threads (such as the "head" thread) cause MySQL to go off and grind for several seconds blocking out all other activity. Accessing this thread also slows down the server much more than those of "normal" length, but the results are far less severe.
If you ever notice sustained periods of slowness outside the backup/admin window (currently midnight-4AM EST) and can rule out network congestion, please feel free to drop me a note (with as much info as you can provide) at firstname.lastname@example.org