Hot water flow is slow from 3 L/3KW instant geyser (hot water heater)
30°C temp raise (10°C to 40°C) for 1L/min requires about 2,1kW of power. So with a 3kW POU you can't expect a big flow. An 'adequate' shower consists in about 6L/min. These are the numbers. Especially if main water is cold, instant POU are not enough, better to install a under-sink tanked heater (10L), it delivers much better flow even at higher temperatures.