Very recently I started to use the very nice Karma Test Runner. I really do recommend to check it out.
However I wondered why tests ran very slow sometimes (>20s) and sometimes they where very fast (<1s). Quickly I realised the issue was having the Chrome tab in the background:
The Chrome tab which was connected to Karma was one among others. I used the other tabs for web browsing and development, so the Karma tab wasn’t the active tab. This gives it a very low priority and running tests became extremely sluggish.
This does not happen if the tab is the active on (= you can actually see it). To resolve the issue, simply open a new Chrome window and open the Karma URL there to run the tests. Tests are fast again and if you want to, just minimize this window so it gets out of the way.
Happy coding!