Inconsistent hot water pressure after install

Hi all,
So we had a new water heater installed on this 16 unit complex and now the pressure is lower on the hot side on all fixtures and it'll be high pressure sometimes then it'll start gradually getting lower and lower as I run the shower significantly lower then it dropped before under high use. Note I did not do this installation and two guys did this installation and I'm just trying to figure out the solution to this problem. I've checked all the basics no aerators are clogged the mixing valve has been replaced no tub spouts are leaking there's no major leaks in the system all valves are open all the way. I can't look at the water heater because it's locked behind a door now but I did see it what they did and I noticed that they put a thermometer on one of the inlet or Outlet pipes and also another fitting that look like it had some type of hot dog kind of end on both sides. They said that they did not reduce the pressure and I'm thinking that the piping was downsized that's reduce the pressure but I'm also thinking that maybe they did not downsize any piping because of the buildup of pressure that I get on the hot water side when I take off the aerator on the sink I can see that there's a lot of pressure. But I don't understand why I'm getting this gradual drop in pressure that significantly drops of hot water pressure to almost or I'm not getting enough water to wash off now. And all the basics of been checked like I said shower heads been taken off and checked even got the most lowest pressure shower head possible to get the most water out of it and I can still notice that I just don't have that powerful pressure jet of water like there was when the old hot water heater was on, and I talked to various hot water heater manufacturers and they said that there's no water saving device or restrictions on on water heaters at all and these guys came in and they installed a pump to to try to relieve this problem which you did pretty much nothing. So I'm thinking that you either the piping was downsized or the thermometer or that other hot dog looking fitting is reducing the pressure or there's a air gap in the system which I have attempted to purge in my unit by just doing the method of turning on the hot water side and capping it off and then gradually turning on the cold water leaving it like that for a while shutting off the cold water and then removing the cap and letting the hot water flow but I thought I had a stable pressure but then it started dropping again like it's getting just too much use to handle the demand of the building but it never had this problem before with the old hot water heater, so I'm thinking that maybe there's air trapped in the system since I'm in an upstairs unit on the very closest end of the loop to the water heater or the pressure relief valve is bad, or the expansion tank relief Schrader valve is bad because this evening are removed my aerator from the kitchen sink and turned on the hot and it blast it out full pressure but then it gradually reduced and reduced. Any thoughts? Thanks

I can't identify that hot dog fitting closest I can come to is a bell reducer coupling, but it looks like a bell reducer union, but I don't even see that fitting made anywhere

The first thing that must be confirmed is that the replacement heater didn't result in any piping being of a smaller size and/or had device(s) added that were not there before.

As far as initial pressure being high and then falling rapidly sounds like you're seeing the results of stored up static pressure(no flow pressure) which can be the results of an expansion tank. And in a case of what apparently must be a sizable hot water system, likely has a sizable expansion tank, contributing to a longer burst of pressure. Then followed by a rapid decrease in pressure due to the friction loss through whatever changed that created the added losses.

Did they perhaps add a thermal mixing valve at the hot water tank that didn't exist before?

A 16 unit complex requires a large hot water supply and flow capabilities. As well as a sizable hot water line.
Did they use more than one unit piped together?

Chances are if there was a change in heater connections sizes, etc, you won't know.

There are so many things that you face inconsistent hot water heater pressure. Water leaving the hot water heater should flow at the same pressure as the cold water in your home and if its not flowing well then it can be heater tank issue or may be installation error. Annual maintenance with the help of experts can fix the issues and make things smoother. However, you can call any local water heater repair maintenance company for quick solution.

They might have installed heat trap style nipples right where the water lines enter and exit the water heater.
Not likely, but they make them.
They have a little captive ball inside that acts somewhat like a check valve, to keep the water heater from radiating heat into the water lines, until needed.
These are often junky, and the little balls inside can fail to move properly.
They also are not a full-port style of nipple, so at best they cut down on hot water flow.