Low shower Pressure???

Delta 1300 drop in Pressure???

I have a fifteen year old Delta shower, it's a single lever that turns counter-clockwise. (It appears to be a 1300 or 1400.)The water pressure recently loss pressure. It is extremely low coming out of the lower faucet, not just the head. Why the sudden drop in pressure?

If it's the pressure balance faucet, then any blockage from either side would reduce the entire pressure.

Removing the cartridge and inspecting may be in order.