Recent pressure drop in entire house

I've got a recent low-flow problem everywhere in my house. I think I know what's wrong but wanted a second opinion from somebody more experienced. The well pressure tank's pressure switch is on a 30/50 setting and the house-side pressure gauge indicates 30-50 psi through the cycle. The gauge and pressure switch are both about 6 months old (switch failed, both were replaced). There's good flow at the release valve on the well side of the tank (not measured, but at least a few gallons per minute) but very low flow at every outlet in the house (measured 0.25 gallons per minute at the kitchen sink, which is the closest outlet to the tank, at the low-pressure end of the cycle). There's only about 12 feet of all-copper pipe about half of which is new between the tank and the kitchen sink, so I don't think it's a pipe problem -- no galvanized or too many 90's or anything like that. I'm thinking that the whole-house shutoff valve is either bad or obstructed. It's old (guessing 10+ years) but still works just fine, although last night when I closed it, it developed a leak at the handle stem that would not stop until I tightened the valve body cap.

If the shutoff is the problem, do you think there's any chance that taking it apart and cleaning it out might fix the problem if it's just obstructed, or should I just replace it? It would really be a hassle right now to replace it...

Any other thoughts -- I am diagnosing this wrong?

Thanks.

Try taking the diffusers/screens out of the faucets and see what happens. Might have gotten some sand/crud in things?