Oof, just had a race that started with 24 players and it was a complete disaster.
The disaster wasn't down to the players - everyone was decently clean and I still came 6th - but the game completely broke down. Just from observations (ie. not real measurements) the game was running at around 30-40FPS, with some spikes to what felt like about 5FPS. It completely messed with me having these stutters, and in some cases I hit a wall or spun out completely.
I dunno if this was just one isolated case but it ran like shit, I'm thinking something needs to be seriously optimised or downgraded if this is a common issue.
I know game clips are 30FPS so you won't see the "general" bad performance, but there are some awful stutters I captured:
http://xboxdvr.com/gamer/TheAdmiester/video/9962756