We have run into some issues with Visual Studio, VB.NET, and large projects recently and I thought I would just share this information with you so that you can take some action on your side. The issue that we ran into is that the larger our VB.NET projects became, the slower the response in the .NET editor as well as Intellisense became. Not only that, Visual Studio would just flat out crash...and many times not save your work. This became VERY frustrating...and I am sure that we were not alone 
Due to our high frustration level, we finally contacted Microsoft and worked through the issue. We were right...we weren't the only people with this issue as they were already in the phase of testing a hotfix. Their initial resolution was to just disable the XML documentation...which just flat out wasn't going to cut it. Though when you did this, the IDE was MUCH faster and the crashes subsided. Today we got a hotfix from Microsoft and so far so good. The Intellisense is much faster, we have our XML docs back, and hopefully this will keep the IDE from dying!
Here was the KB article that was referenced prior to this fix: http://support.microsoft.com/?id=946452 . Even though this article doesn't yet mention a hotfix, one does exist. Just place a call to MS support, tell them your issue, and you should be able to get this fix.