I agree with the others who recommend replacing the water line. I know from experience that this is often easier said than done.
I have a water supply line out to my barn with a similar flow issue. In my case, there is no easy route to run a new line due to concrete patio, asphalt driveway, etc. I used one of these well pressure tanks to fix the problem. No pump is required:
WX-250 - Amtrol WX-250 - WX-250 (145S1), 44 Gal WELL-X-TROL Well Tank (Stand)
The existing low flow line charges the tank to the same pressure as the house and gives me local water storage at around 50 psi. The tank I have is 50 gal which gives me plenty of water for washing machinery. If the pressure runs low, I just wait a couple of minutes for the tank to recharge.
If you have the space for a tank, I think this principle might work for a hot water heater and shower.