Reduced flow from bathroom faucet but not anywhere else
It sounds so similar, practically identical, to the issue that I was having, that I would not be surprised to find out we live in the same building.

Jumping right to solution: I bet that your weak flow problem is caused by debris (probably mineral deposits) in the pipes feeding the mixing valve.

To check the hypothesis without doing any work, attach a water pressure gauge to your problematic faucet. Take a reading with water on, then compare that to a good faucet. If my hunch is correct, those readings will be about the same.

How to fix will depend on particular type of your mixing valve. Shut off the mains, then disassemble the mixing valve. Take photos as you go, this will simplify the job of putting it back together. In my case, there were mineral deposits (rocks) in the pipes, just prior to tiny plastic one-way siphons. Removing that stuff resulted in a dramatic improvement of the flow. I can post my own pictures, but I doubt they will be helpful to you. That is, unless you really do live in the same building.