If your landlord is not on board to troubleshoot and fix his property, you need to find a new place, start barking higher up their tree, or get the city inspection authority involved. But double-check all the valves and aerators and that stuff first. And talk to your neighbors on the same riser. Good luck.

In terms of the worst case ... I had what might have been a similar situation in an early 1970s building: On a given riser, hot water volume was barely adequate if one tenant had the tap on. If two were drawing water, it was a trickle. Due to the usual broken and frozen riser valves, plumbing contractor had to shut off the building. They found clogged galvanized risers and horizontal piping in the complaining units. They had to run new copper in the basement since the old mains were in the first floor ceiling.

Then they ran copper stubs up to first floor and pex from first to third so they wouldn't have to open as much wall. But still a lot of wall had to be opened in the units to get to the kitchen and bathroom of each. I looked at some of the pipe sections they cut out. The 3/4" had sections that were the width of a spaghetti noodle, and the 1/2" horizontal piping, some of it you couldn't even see a pinhole of daylight through a foot of pipe.

This was an all-day job for two or three plumbers, and it was not cheap. I can't remember the cost to get these three units decent, but it was high three or low four figures per unit. And we had construction & maintenance staff hang, tape and paint new drywall, so that work was overhead. So there's two or three months' rent down the drain.

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Link to the State of Wisconsin Plumbing Code:
[www.legis.state.wi.us]
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I am not a plumber.
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