I use Cornerstone for managing my Subversion repositories, so I don't need Xcode's integration, and I'm noticing Xcode is dumping loads of SVN-related output into my console (it recognises that my project is a local SVN copy)...
In particular, there are loads of ERROR: Your working copy returned an SVN status of "external" that we don't handle (yet)
messages due to my svn:externals.
This has to be slowing the IDE down - does anyone know of any way to disable the SVN integration in Xcode 4?