I have a similar problem to the one mentioned here:
How do I diagnose weak water flow from tub faucet and shower?
The bathroom faucet in my rental apartment has a very slow flow even with both handles fully opened. However, no other fixture in the house is having problems.
Upstream of the faucet each line (hot and cold) has a diaphragm shutoff valve. Upstream of that is the valves for the whole house. Because this is happening on both lines, I'm almost positive this has to be an issue with the faucet it self.
Environmental factors that one might also need to consider:
Other units in the condo had frozen pipes that burst and needed to be shut off. Given the Mickey-Mouse jobs I found in other areas of the house, it wouldn't surprise me to learn that this faucet is fed from a different unit's lines lines.
My renter is a slob, and managed to ruin every appliance in one way or another, so I wouldn't put it past him to have found a way to mess up a faucet. That said, he did say that this issue coincided with a previous pipe freezing.
Best Answer
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.