Last night, I finally found the performance problem in the new debugger code - and it was really trivial:
We were missing a mono_debugger_unlock(), so we got a deadlock on exit. There's a 2 second timeout in mono_domain_finalize() when it's called from mini_cleanup() and we have 39 nunit tests - makes exactly 78 seconds in total.
Things are now fixed and the new debugger code is as fast as the old one :-)