When I start the car cold in the morning and shift into gear, either reverse or drive, the transmission thumps hard into 2nd or 3rd gear. Acceleration is very weak, but drives okay at 25 to 40 MPH. Any faster and the RPMs go way up but transmission will not shift up to 5th or 6th. If I drive at least 10 minutes then turn off the car and re-start, it drives fine. Doesn’t do this every day either. Sometimes is fine for weeks before it does it again.
Have the transmission fluid drained and refilled and the filter changed. I’d have this done at a Kia dealer so you have a better chance of getting the correct fluid.