Napari thread workers lead to massive slowdown

[constantinpape]

I have noticed that napari thread workers lead to a massive slowdown for our interactive functionality. This seems to be a regression for napari >= 0.5; I haven't seen this effect previously. See https://forum.image.sc/t/napari-thread-worker-leads-to-massive-slowdown/103786 for details.

For now I will deactivate the thread workers, see #739 , but it would be nice to figure out why this happens and fix the issue to make the viewer reactive again during the longer running computations.