If it's already synced to MIDI Clock and it's still losing time, it could be a problem with a bad MIDI cable or some such. If MIDI messages are getting lost, the CS2x would not have any way of knowing. Does the CS2x's tempo vary, or is it just consistently slow?
Switches, knobs, buttons, LEDs, LCD screens, monitors, keys, mice, jacks, sockets. Now two joysticks!