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?