I’d check two things right off.
The first would be the body ground off the battery. There should be a heavy connection that goes down to the engine or frame, and a smaller connection that goes to the fender. If the small one isn’t hooked up, some electrical components like the instrument cluster may behave funny.
Second would be to inspect the VSS and wiring harness coming off the transmission. Ensure the wiring harness is not chafed and rubbing on any surfaces.
Since the speedometer is 100% electric, there should not be any risk to operating the car with it doing that, however it would be reasonable to determine why it’s doing that.