I have a duel sink bathroom with identical single handle Moen faucets. They both were working fine then the water flow on one became very slow. I replaced the cartridge but that didn't fix the problem. The hot and cold water are both slow. The second faucet which is connected to the same shutoff valves with a T works fine. What could be causing this.

Have you removed the aerator, (see image) and checked and cleaned the screens? Let me know. Tom

Tom. Please send this image to the last post you did.
Lee, what post would that be?

Have you removed the aerator, (see image) and checked and cleaned the screens? Let me know. Tom

That was the first thing I checked. It's clean.

When you replaced the cartridge did you turn the water back on t6o flush out the supplies and the open valve body? Back to you. Tom

Do as Tom says. Pull your cartridge again and turn on the stops under the sink, one at a time, this should let you know if the problem is in the supply line, valve body, or new cartridge. If you have good pressure at hot and cold, then turn both on at same time to clear valve body. Also, moen cartridges are pretty basic, you can slide them apart and check for any dabree inside. Good luck, and please let us know what you find.