1. Rear hub
To reduce mechanical drag from the freehub, some brands have clutch mechanisms that disengage the ratchet when you’re coasting but re-engage it as soon as you resume pedalling. Zipp’s Axial Clutch is one such system and can be found on its Cognition hubs.
2. Rim depth
Choosing rim depth is essentially a balancing act between speed and handling. A deeper rim should offer greater sustained speed but will be more vulnerable to crosswinds. It’ll also be heavier, and weight is a factor if you’ll be hitting the hills when racing
Unless you’re using disc or tri-spoke wheels, your wheels are going to feature spokes. Round spokes are common but the least aerodynamic. Bladed spokes are more aero but need to be correctly aligned, while ovalised are said to provide the biggest aero benefits.
If you’re using rim brakes, carbon wheels simply don’t stop as well as aluminium wheels. To improve their performance wheel manufacturers add all sorts of coatings and textures on to their wheels’ brake tracks to improve their stopping capabilities.