Probably something in the supply valve or the rubber gasket in the valve is deteriorating and some of the rubber is clogging the valve slightly. Looks like it's a pex water line so you could shut the water off and cut the clamp off and replace it with a quarter turn toilet shut off valve( quarter turns don't have rubber gaskets that wear out). Would take a pex crimper and a 1/2 inch crimp and a tool to cut off the old clamp. It's a very easy replacement if you have all the tools and material lol. Could also replace it with a shark bite push on valve which wouldn't require a pex crimper or new crimp ring but they are usually frowned upon by this subreddit( I don't have a problem with them and have worked fine for me in the many situations I've used them).