Do you use Squeak 3.9a? If so, I think it is partly because of the new TestRunner, which seems to be running multi-threaded now.
No, the old one ran multi-threaded, the new one does not. In the new test-runner the tests all run in the morphic-process and block the whole system while they are running, however they run *much* faster ...
Lukas
-- Lukas Renggli http://www.lukas-renggli.ch