why is it when you turn on the hot tap water it slows (less GPM) as it gets warmer?
Less water pressure. The hot tap water is coming from your water heater, the cold is coming from the pipes in the ground that have greater pressure. You will notice that the water comes out fastest when you are in the middle and the two pressures are combined.
but isn’t the pressure going into the tank constant?therefore same should be true coming out?
Some older washers swell when they get warm, thus reducing the flow. I have heard that you can replace them. Mine do the same thing, but I haven’t gotten around to it yet.
Start with changing the washers. It’s the most likely cause and the cheapest and easiest to fix.
Brass, the most common tap/washer material, has one of the highest expansion ratios.
Unless you have a geyser of water, the gap inside the spigot for the water to get through is very narrow. Hot water expands the internal fittings and closes the gap somewhat. Turning on the tap more will reveal the same amount of pressure as a cold tap would have, so the problem isn’t water pressure.