This is a funny bug! Based on the youtube vid provided above, the bug is activated when the game autosaves during an advanced slow-mo effect. Obviously, the gamestate saved is assuming Geralt to be moving at "standard slow-mo" speed when it's saving, resulting in that value being saved to the autosave file. Upon reloading, the computer multiplies the equivalent and opposite value to your movement speed so that you do not reload in slow motion...resulting in -- Super Speed!!!
I would try to trick the computer into doing those calculations again. Remove ANY skill that affects your speed. Find a quest trigger that will initiate an autosave. Aim with the crossbow and move forward to activate the autosave. This should put the computer through the same paces, and result in your top-speed being permanently lowered. By reloading and doing it again, it should result in the top speed being lowered again -- as well as overwriting the next corrupt auotsave file. Not sure what the magical formula to return to "real speed" would be, if it even works.
Ideally, turning off all autosaving should prevent this problem from ever occurring. Instead of a super-speed glitch, you'll wind up losing 2 hours of progress when you forget to save and a drowner sucker-punches you in the pancreas...
