Tire, hub, cassette, brake rub. But I know nothing honestly, total newby. Except that the disc should not be slower, it seems like there has to be some other difference to account for it and that it's worth sorting that out. I guess the other possibility is that it's somehow affecting your position on the bike, if it handles differently and you are not as relaxed.

Was this just one TT where you saw the difference? Or multiple efforts on the same course?