I think the current speed ( only 6 riding skill ) is fine.

However, is there anyway to lower the weight effect while mounted? Because I think that's going to be one of the main reasons to ride a mount. As currently I can run faster than a mount but not as long (as I have to sprint).

However, I think the main purpose of a mount would be to pull carts or heavy loads, as well as long distance travel. I think with a mount it should have similar effect as using a cart, part of the load is offseting it's weight onto the cart or in this case onto the mount.