I have only noticed slowdowns when there is a lot of stuff on screen, typically at the very start of a track when you have all 8 ships on the grid, but that is over in less than a second and doesn't affect gameplay at all.

I've never experienced slowdown in detonator; and it is incredibly rare in single races. In fact I think all slowdown issues have been online, which indicates it could be a bandwidth issue, which could affect your HDD... but that's pulling straws, and probably isn't the case. NFI, really.

I've been running the game on a Sony 1080p TV over 3 years this month, and I'm usually critical... yet I don't have any significant complaints regarding HD's performance. Oh and, screens don't really dictate any fps drop...