Too little information to say anything. Please provide: Camera serial number Dark frame in FITS format Non-saturated light frame in FITS format
The curved edge of the image was the clue. I disassembled the filter wheel and one filter was unscrewed and stopping the works. It was sluggish before and now jammed. I tightened it up , reassembled, and tested. The filters change instantly where they had been very slow. All is well, as Douglas Adams says, "don't panic."