If your shower is performing poorly, it can be for a number of reasons. Often, low water pressure in your shower is the main cause.
Alternatively, your hot water system may not be able to provide enough hot water. This is a common problem with small combi boilers.
Poorly designed shower valves and fittings can restrict the flow of water too. Such fittings need more pressure to drive the water flow out of the shower head.
Another factor can be limescale deposits on the shower head. A simple limescale remover may significantly improve the performance of your shower.