And i thought that everyone wanted to maximise performance, not nobble it.

The greater the electrical drain to run these heaters, the greater the LOAD on the altenator, & guess what makes the altenator turn. Right,...your engine. It takes power from your engine to drive the electric heater.
How much power? I don't know, but with your engine idling, flick your lights on & switch to high beam. Notice how much the revs drop. They will drop proportionately more as the electrical load rises. Water heating is FREE as far as the engine is concerned. The greater the load that an engine carries, from any source, like aircon, the more fuel it uses to deliver a specific performance.
Free sounds like the right price to me