First off, you are getting defensive over something that is rather trivial.
Secondly, I even stated in my own post that it does not seem to be an option and explained why, given that XP Mode is, in itself, an application. This application is limited to a situation where in your own system specs, you only state Windows 7 Beta. Nothing about memory, whether or not you are using Windows 7 x32 or x64 until now.
These two factors are important to note because in either case, assigning higher priority to it will not work because you are adding another layer to get a program to work. This layer will be drastically affected not by priority but whether or not it has to juggle its resources along with the resources the application it is trying to help process in a possibly restricted environment, namely, if you are in a x32 setup with 2 gigs of Ram, that is going to require juggling of a lot of resources which means that the XP mode application will be lagging because it will be doing VM swapping along with the program it is trying to assist.
Furthermore, even using x64 with 4 gigs of ram, you still have that problem because in the past, I have also seen Windows XP with 4 gigs of RAM still do the occasional virtual memory swapping and Windows XP x32 only sees 3 to 3.5 Gigs of RAM natively. In Windows 7 x64, at 4 gigs, it sometimes reserves some memory that you may still do a bit of Virtual Memory swapping still.
As stated, assigning higher priority under those conditions will not solve the problem. And his answer stated one of the realities of the situation.
The obsolesce of Windows XP, while it might not be 100% accurate persay, does not mean that for one, its end is not coming soon as noted here:
Microsoft Support Lifecycle
The recommendation of dual booting the system is also valid if you are looking to eliminate the lag all together, as you are not relegating yourself to that extra layer of an Application trying to grab resources ontop of the resources the actual application you are running needs.