I've noticed on my 1971 Zorki-4 that the rear curtain travel at speeds of 1/30th and lower appears to be slow. This is especially evident at 1/30th. Sometimes it looks ok but more often than not it seems slow. I tried adding a half turn of extra tension on the rear curtain spring but that did't seem to affect the travel. Any suggestions as to what to check?
The camera needs a CLA. Changing curtain tension from the proper settings to compensate for bad lubricants will just cause other problems. Sorry there's no quick fix.