megaflo pressure

Can anyone please tell me why when I open a cold tap, the pressure drops noticeably on my shower; however, when the hot tap opens, there is less of a change. If I open both the hot and cold tap in the bathroom, the hot is noticeably faster running than the cold. I have a water softner fitted with a highflow kit but even if I bypass that, the cold isn't as good as the hot. There is 7bar coming into the house, reduced to 3 at the megaflo and I am getting approximately 20-25 litres per minute out of the shower. Is this normal or does something sound not right? Many thanks in advance.
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