You didn't mention which version of Visual Studio you are running, but I found a blurb that claims they did some performance improvements between 2005 and 2005SP1
We made a change in the VS2005 SP1
(and included it in VS 2008) which
optimizes performance in web projects
by first performing a lexical search
of the element being refactored to
determine if the page needs to be
loaded. I apologize for it taking us
so long to update this issue with the
resolution; there were several connect
bugs that were related and while we
updated most we managed to miss some.
We expect that in the future we'll do
even more performance enhancements for
this scenario (post VS 2008), but
hopefully the optimizations we have
done drastically increase performance
in most cases.
Also, this solution kinda sucks, but MS offered this workaround too:
With respect to workarounds you can
increase the performance fairly
significantly if you open all of the
web pages prior to performing the
refactoring. I agree that it isn't a
particularly good workaround, but for
VS 2005 we chose to prioritze
correctness over performance for
refactoring.
Source: https://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=92239