The most common reason that both the hot and cold water are reduced volume in a single faucet is a clogged aerator. It is a simple thing to check and most likely you can avoid calling your plumber. http://www.dummies.com/how-to/content/how-to-unclog-a-faucet-aerator.navId-323582.html Here is a site that will show you how to clean an aerator.
Also if you have a pullout sprayer as part of your kitchen spout, where the hose connects to the sprayer there is also a screen in there. Disconnect the hose and clear any rust out and this should solve your problem.
The aerator is the part of the faucet that the water directly comes out of. It is a small device on the very end of the faucet spout. If you cannot locate the aerator please give me the brand and model of the faucet and I will find a picture to help you locate it.
There are other things to check, that are more involved but this is the easiest thing to fix. If it doesn't seem to work I will be happy to help you further and we can go to the next step. I would recommend watching the video I included with my first answer as it will show you were an aerator is. If you have already cleaned this please give me the make and model of your faucet because the next step is taking the faucet apart.
Mike, I aready replaced the cartridge and cleaned the screen that is at the end of the faucet. The Faucet is a Price Pfister that has the pull out sprayer with a push button on top for both spray and straight flow. I checked that to and the part under the rubber facing worked fine when I pushed on it up and down.
There is a second screen as well. Where the hose meets the actual sprayer. If you haven't cleaned this one out I would suggest you do so. http://www.pricepfister.com/www/ImageAssets/mirror/model/kitchen/bixby/bixf538-5lc/33212-0100.pdf Go to this site and look at step number 16 on the instruction sheet, it doesn't matter if this is your exact faucet because this will just help you locate where the second screen is. It would be inside 16A right before 16B connects to it.
Ok, Are both the stream and the spray functions subject to low water flow? If so the it is either in your faucet or further down the line ( in the supply tubes or piping. ) The way we would check this is to remove the cartridge with the valves under the sink off. Then slowly turn the valves on one at a time(hot then cold or vice versa). You have to be careful doing this however because water should shoot up. If it doesn't shoot up or sort of dribbles out, your supply tubes or angle stop valves are clogged.
To test that you will need to remove the supply tubes from the faucet with the water off, then turn the valve back on and verify water flow. If there is water there good pressure, then debris is in the faucet somewhere. If the flow is still compromised you will need to take the supply tube off completely. Turn the valves back on slowly and verify flow. If you get good flow out of the valves directly, then you have to replace the supply tubes.
The only way to find where the problem is, is to work your way back words until you find where the clog in the line is. If you need clarification on any of these steps, or if you have any other questions, please feel free to ask.