This issue is usually either an problem with the control stick, or the result of trying to calibrate the controller using the "hidden (secret) menu". But let's try to narrow it down a bit:
1.) Does the "slow" apply to all sticks (vertical, forward, sideways, yaw)? (this is the main clue)
2.) Have you checked stick/slider response in "Hardware Monitor"? (Especially "K3")
3.) Have you attempted to calibrate the ST16 (hidden menu) recently?
4.) Have you been in "Channel Settings" recently? (Even if you don't think you changed anything,)