OPen it full, and measure how much comes out in say 30-seconds, then double it. Newer vanity faucets are limited to 2.2gpm (or less). Some faucets have screens on their inlets. Take the supply line off the shutoff, and run that into a bucket to see if that has better volume. if the faucet has removeable supply lines, remove it from the faucet and see what volume there is there. The shutoff valve, if a multiple turn, might need replacing (it's often easier than replacing the washer). If itis the source of the problem, replace with a 1/4-turn one...no washer to fail.