The speedup comes at the cost of the decreasing the cross-sectional area of the flow. The input is 2mph x [big cross section], the output is 15 mph x [small cro

The speedup comes at the cost of the decreasing the cross-sectional area of the flow. The input is 2mph x [big cross section], the output is 15 mph x [small cross section]. So if you take the cross-sectional area into account, the speedup does not violate conservation of energy.