There are several reasons this could be.
Did you have the plumbing worked on prior to this happening or did it get this way gradually over time?
Try removing the strainer under the tip of the faucet and cleaning out the screen real well and see if that might solve your problem. it should screw off but might be difficult enough that you need to use a wrench or channel-lock pliers.
If that doe s not help, turning the stops (Shut offs) off under the sink, and removing the supply line from the stop. But a bucket under the stops to catch the water that drips out before you do this.
With a cup upside down over the top of the stop, gradually turn it on and see if your water pressure=is good at that point. Do both the hot and the cold.
if it is good under the sink there, you will have to disassemble the faucet (as if your were replacesing the washers or cartridge) and clean that out. If it is all corroded inside the faucet, replace the faucet.
If the pressure under the sink was not good, replace both valves.
If after replacing both valves, the pressure is still bad under the sink, you will probably have to consult a plumber who will be able to help you trouble shoot where in your house the restriction is happening.