Perhaps it would be helpful to look at what happens to the electrical energy that enters the pump. Essentially it gets converted to mechanical energy. In the process, there is some inefficiency. The motor consumes more energy than it delivers to the shaft. This inefficiency shows up as heat and in a submerged pump all of it enters the water. With an inline pump, almost all leaves via the surface of the motor housing. The "useful" energy leaving the motor via the shaft drives the impeller. All of this energy goes into the fluid. Some is in the form of a pressure rise and the rest is thermal. Without improving the pump efficiency there isn't much you'll do about this inefficiency.
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