These numbers may not be up to date, and moreover given for a spacious submarine. Modern spacious submarines have a torpedo transfer system which stores torpedoes in a room, access it, carry it to the torpedo and act as an automated arm/winch to ease the process for the crew.According to U.S Naval İnstitute ‘’ A proficient crew can launch its first four weapons in about two minutes. To reload each tube takes 10 to 20 minutes—up to an hour for all four—depending on the arrangement of the room’’
For small and generic sized submarines with berths located at the fore section, i can't even imagine how long it would take for a single tube.