How fast could these submarines go?

Top speed on the surface was 20 or 21 knots, about 23 miles per hour. However, this was an inefficient use of fuel. In WW2, boats departing from Pearl Harbor, Hawaii would usually run at top speed to locations such as Midway Island or Johnston Island. There they would top off fuel and then proceed at efficient speeds (10 to 12 knots) to their assigned patrol area.

They were much slower under water.  Top speed submerged was 9 or 9 ½ knots or a bit over 10 miles per hour. However, top speed would have been limited to less than an hour before the battery is completely drained of power. The more common speed submerged would have been 2 to 3 knots. Higher speeds might have been used to gain attack position. Higher speeds could have been used to try to escape when being attacked but that usually made more noise. Slow, silent and clever was more effective.

The ship’s battery was usually rated at 48 hours if the boat was going 2 knots. The ratio of speed to power needed was cubic. Generally, if the speed was doubled, eight times the power was needed. If the speed went from 2 to 4 knots, the battery would last approximately six hours. Double it again, and the battery likely won’t last an hour. It was called the “one-hour rate.”