Yep. I was afraid that would be the answer. I was hoping that since the fast speeds work fine there may be a simple fix to this, it looks very clean inside, no dust or gunk. I hesitate to do a complete CLA myself yet. I do have a 1962 Zorki 4 I got for $15 that I have been using as a training tool. The front curtain ribbons have come loose from the drum. I have it disassembled and am learning from it but I hesitate to screw around with a mostly working camera just yet. What specific points affect the curtain travel speed and why does it only affect it at slow speeds and not fast speeds? The return spring is the driver and it is tensioned the same regardless. Once the curtain is released to return, it should travel at the same speed from what I can deduce from my "parts" Zorki.