Does a Zenit have a cloth shutter? If so, I also think these are some pretty serious pinholes in the shutter curtain.

My theory is that the moment the mirror swings out of the way (and the curtains are still stationary) the "holes" are burned, and then, as the curatins start to accelerate, we see the "smearing" for a short time, until they move too fast to have an appreciable effect on the film anymore.