Water pressure in bathroom sink
If it's only that one sink it very likely is caused by corrosion or blockage between the shutoff and the faucet. The hot water side is especially susceptible to scale buildup. There are a few basic things to check:  


First make sure the hot shutoff valve is fully on.     
If the shutoff is old it could have scale buildup in the shutoff valve. Turning it on and off a few times will sometimes free it up.


If the hot is a separate faucet:   


Remove the aerator from the faucet. Clean the screen of any debris.
Turn off the hot shutoff valve and remove the hot water cartridge and check it for scale buildup.  


If there is still a flow issue I would replace the shutoff valve and/or  the hot water cartridge starting with whichever one looks most suspect. This should solve your problem.   

Edit
Another possible cause, but less likely, is galvanic corrosion which happens when two different metals are connected such as galvanized pipe coming out of the wall connecting to copper. This requires a special type of connection (dielectric union) to prevent corrosion.