We installed a new faucet about 2 years ago and all was working well. It is a Krause single side handle. Recently we noticed that the flow rate had decreased for the cold side. After a few more days the hot side also had decreased. We closed and opened the valves under the sink and it actually decreased the flow more. Now the cold side is just a little dribble and hot side is a little better, but not like the flow we had initially. We checked the faucet head to see if there was any blockage, but nothing there. What could be the issue here and how to fix?
Almost anytime you run into low flow in a faucet it's caused by clogging somewhere between the shutoffs and the aerator. Sometimes it's calcium buildup and sometimes with a multi-turn shutoff valve it's the rubber washers in the valve that deteriorate and occasionally break apart. Since the flow is different hot to cold I'm guessing the issue is in the shutoff valves. In any case it's easy to check.
If the problem is caused by the shutoff washers breaking down you may not be able to close them fully in which case you'll have to close it at the main. In that instance I'd suggest you replace the shutoff valves with quarter turn ball valves. Next connect your supply lines. If you're reusing the old ones test the water pressure through each line before you reconnect it to your faucet.
The next step is to clear debris from the faucet. Remove the aerator and clean out all debris. If there is calcium buildup use a decalcifier. Vinegar is a good one. Reconnect the supply lines to the faucet and flush the faucet without the aerator to remove any other debris that may be in the line.
If there is further blockage it may be in the cartridge. Turn off the water at the shutoffs and remove and clean the cartridge, Keep in mind that on any modern faucet you won't get the same pressure from the faucet that you will get straight from the supply lines when they are disconnected from the faucet. This is due to federally required flow restrictors built into all new faucets (last 20 years or so).