JScript performance in IE 8 on XP & Server 2003 vs Vista & Server

[S@discussions.microsoft.com Lyle S]

I am currently developing an AJAX intense application. I have noticed that
the first time the application is accessed with IE 8, and some operations are
performed, that performance is pretty quick, however, it degrades fairly
rapidly after a bit of javascript which renders some search results from a
server XHR request. Refreshing the browser (or browsing away) and hitting
the application again does nothing to improve performance. The process needs
to be shut down and restarted. Memory does not seem to be a problem and it
does not appear to be a memory leak issue.

Investigating this, it appears that it is only on XP and Server 2003 that
exhibiting this behavior. Vista and Server 2008 do not degrade like this
with IE 8.

I have completely updated via "Windows Update", reset IE's settings via
"Restore Advanced Settings" and "Reset..." options in the advanced tab,
restarted the systems in "Safe mode with Networking" and lastly tested with
the "IE8 windows XP" image available from microsoft's website, and they all
seem to exhibit this degredation.

Does anyone have any idea at all as to why IE 8 on Vista/2008 would have
such better substained performance than XP/2003 and how it would be addressed
short of upgrading?