Switching between tabs in the editor takes > 1s (on a core i7 with 12GB). That is enough to make it almost useless for me. IntelliJ community edition is much snappier.
I remember the Google team being shocked (as was the Eclipse community) when Juno came out since it was so awful ui-performance wise. They rushed to donate some machines to re-enable performance tests which had been commented out due to lack of resources, it was small like 10K commitment or so. With their help the Eclipse team was able to mitigate the problems, but its still very slow compared to the 3x branch. In a way they seem like they dont want to admit it and defend the 4x branch. I'm sure this Studio release will be a wakeup call.
I suspect we're seeing the result of Eclipses 4x failure. The only light at the end of the tunnel is that Eclipse releases every year along with its huge plugin ecosystem. They almost never miss a date which is amazing considering its usually millions of lines commited on everything from graphical modelers through c++ tools. Maybe the next major release will fix all this crap and restore order to the Force.
edit/tl;dr: 4x performance issue fixes hopefully done by June 2013: https://bugs.eclipse.org/bugs/show_bug.cgi?id=393429