VS2012 is slow for me, but that is because of the TFS Integration. Something about the new Team Explorer is blocking VS2012 from coming up.
For example, I waited 1 minute after trying to open my project in VS2012, then opened the same project in VS2010. VS2010 came up first, I made my changes, while still waiting for VS2012 to open up. Since my VS2010 comes up with Team Explorer visible, once that was available in VS2010, I saw both VS2010 Team Explorer and VS2012 UI flicker at the same moment. Then VS2012 started on the project.