Suggest you go and look up the specific heat of water, do a temperature test of the incoming cold and the outgoing hot, a flow test at the shower head, and see if everything ties up for a 10.5kW shower.
If it does then the problem is with your expectations.
If it does not then there is a fault with the shower.
It shouldn't take more than an hour or so, including learning about the physics of heating water.
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