For maximum speed and a super-plush, smooth ride, stick some of the biggest wheels you can on your longboard! At diameters bigger than 74mm, longboard wheels are unbeliveably fast and smooth, with very high roll speeds and the ability to roll over almost any road or pavement surface you will encounter.
Wheels this big are great for Downhill setups, but are also good for cruising and long-distance pushing longboards. You'll need to consider wheelbite - in order to keep your trucks loose and your deck low to the ground, look for a deck with big cutouts, wheel wells, or a drop-through truck mounting.
Look for a large core in bigger wheels - this increases roll speed even further, and helps the wheel slide better by increasing the rigidity of the wheel and keeping the weight down. Many modern Downhill Wheels feature some of the widest cores going for these exact reasons.