[PappyCain]

A very good question! Yes, to maintain neutral buoyancy and not use the ballast to trim constantly one had to keep the boat moving as you well state. As to speed it was slow to avoid the periscope making a noticable wake and best done with the sun at your back. That made it harder to spot too.

(Side note: when a torpedo is launched, the weight of the boat had to be immediately balanced with ballast, as the launching of a torpedo caused more than a ton of positive buoyancy. If this was not offset by ballast, the boat could pop to the surface, giving away its position. Many U-boats were caught and abruptly attacked due to this simple error.)