A toilet that takes a long time to fill after flushing almost always has a failing fill valve. Replacement takes about 20 minutes and the parts cost under $15.
Turn the shutoff valve behind the toilet clockwise until fully closed. Flush to remove most of the water. Use a towel or sponge to soak up any remaining water in the bottom of the tank.
Disconnect the supply line from the bottom of the fill valve under the tank. Hold the valve body inside the tank and turn the locknut at the bottom counterclockwise to remove it. Lift the old valve out. Set the new Fluidmaster valve into the tank opening, adjust the height so the critical fill line marker sits one inch below the top of the overflow tube. Thread the locknut on from under the tank and tighten hand-tight plus a quarter turn with pliers.
Connect the refill tube from the fill valve to the overflow tube. Reconnect the supply line. Turn the water back on slowly. Let the tank fill and flush once to confirm it refills quickly and shuts off cleanly.