that's combis for you
They can deliver X amount of hot water, heated by Y degrees
Or 2X amount of hot water, but only half a Y.
You won't notice it running a sink or basin, because the litres per minute through the tap are quite small. But filling a bath, you may demand a large flow or water.
For example, e.g. a 24kW combi may deliver about 10 litres per minute of hot water at a temperature rise of 35 degrees C, but a 40kW combi (rare) may deliver 16.4 litres per minute at a temperature rise of 35 degrees C.
If, in midsummer, the incoming water supply from the main was 15C, a 35 degree raise would take it to 50C, which is quite hot.
but if in winter, it was only 5C, a 35 degree raise would only bring it to 40C, which is not very hot.
So turning down the hot tap means that the same amount of heat per minute is delivered to a smaller amount of water, thus heating it faster.
Like a half-full kettlle boiling quicker than a full one.
Sometimes, bathtaps are restricted to reduce the flow of water and hide this effect.
Hot water cylinders work differently.