If it takes longer to heat then it's only because less energy is entering the system,(i.e., electrical wattage), right. The element gets too hot, the resistance increases, the wattage goes down.
I guess I would say that the WH effectiveness or capacity goes down rather than its efficiency. E.g., my truck won't haul as much as it used to because the springs are worn - that's not a loss of 'efficiency'. Semantics I guess.
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