Unfortunately the game is running at about 15fps and is unplayable. All seemed good at the start, but once i got to the showcase Mustang vs Mustang it all went horribly wrong 
Exactly the same here. Have you sent a report to Microsoft via the Xbox One console?
I dropped a ton of frames traveling back to the Hub in the Mustang too.
Same thing was happening to me continuously, had to turn it off because it was completely unplayable.
Hard reset. Game was making me sick to start as well