Before upgrading to 3.1 we use some of EAP's pre-3.0 version.
With that version we run VS2008 solutions using sln2005 runner and set required system variable for framework, and tests was on NUnit 2.4.x. It works almost fine - runner says about problems, but worked - with only minor problems like red lamp during the build.
So today I try to upgrade to 3.1 and find very interesting thing - with sln2005 runner practically nothing changed - but when I switched to sln2008 and set NUnit 2.4 as test framework in settings build became slower in dozens of times.
Logs not say anything helpful to me.
May be it's the same problems as in http://jetbrains.net/tracker/_htmlTemplate/Issue?issue=25-4617