Double-check that the cut-off valves are fully open.

Is the shower valve thermostatic?

If so, try turning up the temp on the heater so the valve is mixing in more cold. Thermostatic valves do not work very well unless they are mixing enough cold water with the hot. If it takes 95% hot and 5% cold to achieve the desired temperature, the resulting flow rate is rather low. That has been my experience with Hansgrohe thermobalance valves

They don't have the same problem mixing in minimal hot with the cold, the flow rate is strong in that case.

Thank you for the replies. The cut off valves are fully open. Yes, I believe the valve is thermostatic. I think the hot water is set fairly high now, but I will double check. But why would turning up the water temp increase flow? I am sorry but I am confused how that would work.... But, I am willing to try it. Will try it tonight. Thanks.

Ed

I'll defer to those more experienced, but I would be inclined to remove the cartridge and inspect for debris. You might be able to blow some bits of crud out of the thing.

Since you recently did the plumbing, this is probably the problem:
The thermobalance cartrige has an inner screen that needs to be cleaned. It is not the same as removing the cartrige. You don't have to turn off the water supply. You don't have to remove the trim plate, just take the valve handle off. The brass body of the cartrige has hex-shaped places so it can be taken apart with 2 crescent wrenches. The male part has a screen around the base that gets clogged up with debris. More debris comes through the hot water pipes than the cold, so the low-flow issue manifests itself exactly as you describe.

I clean my valves a couple times per year. When you go to put it back together, don't tighten it too much. It is a precision-fitted union with rubber gaskets. Only snug-tighten with the crescent wrenches to avoid damaging the soft brass.

I have always had low flow issues with mine unless the hot water is sufficently hot that the valve mixes in some cold water too. Since you say the water is already plenty hot, this is not the issue.

Thank you. I will check this when I get home in a few days. Thanks again.

Thank you! I replaced the shower valve and cleaned out the rough valve area and presto great shower. Thanks again!