I replaced a gas hot water heater. I opened the valve to fill it while the faucets were off.( The gas is not yet turned on.) Then opened the kitchen hot ,of course a blast of air then some slightly cloudy water came out but the flow on hot is less than half of the flow from cold from the single handle faucet. I then opened the hot valve for the washer in the utility room near the water heater and it seems to flow about the same as the cold valve next to it. It appears the 25 ft of 1/2" copper pipe( for hot)to the kitchen, in the concrete pad, is restricted. When I replaced the copper flex line out of the heater ,it had a spoonful of white scale in it. I am thinking the pressure changes caused loosened scale to collect at some point. Any ideas ?
I would disconnect the supply at that isolation valve and then "backfeed " it to try flushing the scale out the way it came in.
Even feeding both points at the same time may make a difference.
Did this for one of my taps and it worked...
Have you checked for any of those tiny inline scale filters - common before washing machines etc.