How fast do lowrider bikes go?

I'm assuming here that you're talking about a cruiser style bicycle.

First off a bicycle doesn't anywhere, at any speed without a rider. And a strong rider can do crazy things even on the most inappropriate bicycle. Likewise a weak rider will struggle even with the best bike.

With that said, cruiser bikes aren't built for speed and efficiency. Expect 10-15 MPH on the flat with reasonable effort.