Ok, this is pretty specific, but it's something I've noticed and is a bit annoying since swapping from Win2k to WinXP. When I alt-tab and read forums, my in-game framerate(or whatever) drops just a bit, to the point where I go slower on a mount than normal, and will lag behind someone that I'm auto-following. Or flightpaths will take just a bit longer than they should, that sort of thing. Is there some sort of setting in WinXP to change the way multi-tasking behaves, or something? For reference, I use full-screen mode. I'm aware that this probably would not happen if I played windowed, but I prefer full.

I think you misunderstand, this is not a memory leak situation. Its simply, while I am alt-tabbed out, my WoW runs slower than it normally does. Meaning windows is not giving it as much attention as it needs for certain things to run at full speed, like mounts and flightpaths. When I tab back in, everything is fine again running at 50+ fps.

I often alt-tab while I'm on a flightpath, or if I'm on my mount running in one direction for a long time.

Something I know nothing about, but I did read about in that thread was priority and affinity. Would changing the priority of the process improve my issue? Is changing the priority of the WoW.exe process permanent for that process, or would I have to change the priority every single time I ran the game?

Every time.
You could however utilize the 'start' command to run WoW.
Just make a shortcut or batch file with "start /abovenormal WoW.exe" or "start /high WoW.exe".
No Launcher this way though.

From an alternate perspective:

The proposed solution for high process priority may make no difference. The purpose of setting a priority on a thread is to tell the OS scheduler how it should rank a thread for the distribution of handing out timeslices. So, to be clear, a process priority is a hint for the OS, _not_ for the "game" itself.

Why this distinction? A game engine is a giant loop that runs as fast as it is allowed to by the OS. If the game choses to yield to other processes, in the case that it is in the background, than upping the priority doesn't necessarily yield higher frame rates. It is very likely that if the window is in the background, WoW is opting to yield processing time to other processes (via Sleep, or what have you). IF this is the case, upping your priority will not yield the desired results.

It is generally the case that upping process priority has a marginal impact on performance, for the above reasons. A case where it _would_ increase performance would be if there was a large number of processes at a normal priority, and you started WoW with a 'high' priority. The OS scheduler is still going to pass timeslices to that 'large number' of normal processes, they will just get less favored than the high priority WoW.

The real oddity here is that you go more slowly, in-game. In a typical client/server model, the server is authoritive and the client sends update information which is processed by the server, and the server corrects any interpolation errors the client makes. For example, people warping means that your client thinks they are in one place, probably due to inconsistent updates for that client, and your client interpolates those values as best it can. When the server sends an update which is significantly different than the values the client is interpolating on, warping occurs. The server is, in essence saying, "This object is at location XYZ," and your client will simply place the object there (or interpolate it to that position over a very short value of t) and poof, warping.

With this in mind, it is very odd that a flight path would take longer due to framerate issues.