I cannot be sure, since I am also lucky to use a new version. You probably have a huge test suite with lots of tests, aren’t you? If you do not need application domain isolation for other reasons, you can turn it off and then you save the remoting/marshaling time.
Try this please and give feedback to us how it went:
nunit-console.exe /noshadow /nologo /process Single /domain None whatever.dll
There is also a way to add these options via the configuration files, if you do not want to touch your build scripts even with a stick. 🙂