Here's a shutter tester that has served me well and should work with the Zorki 6 since it has a hinged back.
Sometimes you might see "double slits" which is fine, the most improtant thing is to observe the width and how it gets wider or narrower as you move up or down the shutter speeds.
If you determine the second curtain is slow [usually evident at "B"]. You'll have to remove the top, here's a good URL that illustrates how to do it: http://muj.proit.cz/roman/z6disass/
Scroll down to the second to the last picture, observe the shutter speed disk area and if it is dirty clean it up with lighter fluid and blow excess liquid and grime with compressed air. Try working the shutter again at "B" if there's some improvement but still not "snappy enough" add a few drops of watch maker's oil underneath the shutter speed disk particularly the pin that rotates to the right as the second curtain closes. Assuming the curtain rollers are not corroded, this minor CLA will restore the camera back to shoot print film with reasonable accuracy.
Hope this helps and good luck!