Doug, if you hooked up a hose to the shut off valve under the sink, how long will it take to fill the can? This will eliminate the new fixture and start locating the trouble. Could be there's a restriction in the water pipe between your bathroom and kitchen. If perhaps a upstream valve decided to come apart some time ago, a piece could be caught in a downstream elbow or at the shut off valve itself. How easy would it be to replace the shut off valve. Do you have the same flow rate problem with hot water too?