Significant editing slow when test window is open

I could not figure out for a while why sometimes editor response slows down (a couple seconds lag). Finally, I noticed that it happens after I run tests and test window (4:Run) is open. When I close the window responsiveness improves till I run the tests again. Does not make much sense why that would happen but it is very consistent (IU-129.713 / 0.14.114).