Our rental house was built in the 1950s and they did things a little different then things are done today. We noticed that the drain in our shower was draining very slow. After a plumber came out and snaked the drain and the vent, things didn’t improve. The plumber said that the drain was a 1.5 inch pipe whereas most drains are 2 inch.
So what were we to do? Every time we took a shower, the water would fill up the basin (it’s a small shower). One day I decided to muck with the shower head and reduce the water flow. Once I did this, I noticed that the shower didn’t fill up as quickly. I mentioned this to my wife and she said that if she turned the handles such that less water was coming out, she saw the same thing. I adjusted the shower head a bit more and now we no longer have to shower with a few inches of water at our feet.
This solution, of course, is pretty obvious. In order to reduce the output, decrease the input. Why I didn’t think of this earlier, I have no idea.