Visual Studio 2005 does a lot of things in the background, including continuous HTML validation. This can really drag down the system and make editing a pain, so Scott Guthrie has offered some tips.
The first thing you can do is to install Service Pack 1. According to Scott Guthrie, they have done some performance tuning so that the HTML validation works better when editing large documents.
If you still have having problems, you can turn HTML validation off. Unlike the pre-SP version of Visual Studio, the IDE will no longer turn it back on automatically whenever is finds an error that prevents you from switching to design mode.
Finally, you can turn off property grid support . For those of you who aren’t familiar with this, Visual Studio updates the property grid as you move from tag to tag. If you are the kind of developer who does all of your HTML editing in text, the grid updates are just a waste of cycles.