It may be because of cold water - the shower stalls until the desired temperature is reached. Good to begin with because the temperature of the water entering will be that of the house - then as the flow continues much colder water enters the shower and needs more power to warm it up.
Your plumber(s) may neglect to explain this - they get much more money if they install a new shower for you.
Does this problem happen only in the winter?
Your statement - There is no difference if I am using other water appliances - is confusing.
I know nothing of showers - I just guess. I think the shower(s) are limiting the flow rate due to the difference between the temperature of water coming in and the temperature required for the heated water going out.
Stay clean...
January 2016