There are a lot of new houses now being built with high pressure mains fed hot and cold water systems in. The two major benefits being a dry loft space (no tanks) and high pressure showers etc. They are good systems but rely on good pressure and flow from the incoming mains water supply from the street.
If these systems aren’t balanced properly with the correct choice of taps at each fitment then poor performance will be experienced when a number of outlets are used simultaneously. However; in some circumstances it is possible to imrpove things by taking a pressure and flow reading from the main stop-valve at the point of entry then taking flow readings at each tap and fitting flow regulators to each tap.
Lack of flow can also be improved with the addition of an accumulator, a large water storage vessel with a pre-charge of pressure stored within. However; accumulators are quite large and can be as big as the actual hot water cylinder or even larger.