I had a Felt B2 around the turn of the century. 650c and a 58t large ring. Definitely fast. Any faster than another aero bike on 700? Probably not.

The fastest splits I ever pulled off in a tri were on a regular road bike with a disc wheel and clip ons.

Anyway, the theory was that they're more aero being closer to the ground. Or something.

I think the only place it still exists is with short women's bikes.