This is one of the easier pool issues to diagnose and repair. It normally arises around spring when debris is plentiful. If your pump basket has holes in it or is incorrectly installed, debris can get around it and become clogged in the impeller’s vanes. Once enough dog hair, pine needles, and grass clippings get past, the pressure rate will drop and the flow will reduce.
After checking to see that the basket is not full and determining that the motor is not making an unusual noise, switch off the power and clear the impeller. It may be necessary to open the pump at the seal plate to get to the impeller. If small rocks and seeds are the issue, you can use a curved piece of wire to remove the debris.