Tri bikes use carbon fiber wheels that are lighter and have less wind resistance. Here's a website that sell a few different types.
Wheels | Zipp - Speed Weaponry
I have heard stories that the full carbon disks do not do well in side winds, causing the bikes to move all over the road. Could be a safety hazard.
I use Cane Creek wheels, nice aero shape, not too extreme.
The type of wheels makes a huge difference on top speed.
I would like to see a bike made without spokes, that would just have a rim, open in the center.
That would clean up the airflow considerably.