I have CITY water. When I turn on my garden hose I have plenty of pressure, but after a few seconds the pressure drops substantially only outside
There is a substantial restriction somewhere - partly-closed valve, over-soldered fitting, something like that. Your 15 seconds or so of good pressure is probably from the hose itself storing water - the slow trickle past the restriction gradually pressurizes the hose to full line pressure - as soon as you start using water, the hose contracts, and when fully contracted you are back to the flow past the restriction.

Check any valves on the line to the hose fitting, first. If it is an over-soldered fitting, those are somewhat of a pain to find, as you basically have to disassemble the pipe to look for them.