A few months ago, out of nowhere, the kitchen sink's hot water pressure became very low. A plummer came over and all he did was totally replace the faucet; nonetheless, the problem still exist so I doubt it's the washers or simply being clogged. I don't exactly trust this plummer; he's saying it might be the pipes outside, but I highly doubt it considering all the water pressure in the house is perfectly fine. Any ideas?

Well, the water pressure in the house was perfectly fine until:
We had a leaking faucet in our bathtub and this amazing plummer took the handles off and replaced the washers. Now the handles need to be turned about a quarter to a third of the way at least for the water to even begin to come out. Additionally, the water pressure is extremely low on both hot and cold. I'm guessing I just need to take it apart and adjust the valves? Any suggestions?

Thanks,
Steve

Hey Steve,
Let's take it one at a time. The kitchen first. It didn't sound like a new faucet was needed or called for. It sounds more like you've failed to flush your heater on a regular basis and the mineral buildup got sucked into your pipes and faucet. All the plumber needed to do was to open up the faucet, clean the areator and inlet ports/seats and flush out the hot water line.
Now you're going to hafta do it. First unscrew the aerator and check and clean the screens. That might be all you ave to do but if that don't get it shut the water off at the stops in the cabinet and open up the faucet and chec kand clean the inlet ports if a one handled faucet or the seats if it's a two knob faucet. Now place pan over the open faucet andturn the hot wate on for a moment to flush out the supply. You should have a full stream, if not click on back. The same for the upstairs faucet. Clean and flush.
Now let's turn our attention to the hot water heater.
For long life and fewer troubles you should keep your heater clear of mineral build-up by flushing on a regular schedule. Let me show you how. Attach a hose to the boiler drain at the bottom of the tank. With the pressure on, open the boiler drain and let it run until the water runs clear. You will see a spurt of red,(rust) followed by white grains,(lime or calcium carbonate). This shouldn't take more then a few minutes. Do this monthly to keep it clear.
Let me know how you make out. Regards. Tom