Why Is My Toilet Flushing And Filling Slower And Slower?

If the bowl is filling only half full but the tank is full , look at the refill hose and make sure it is in the tank filler tube. sometimes it can slip out and the tank will fill up and the bowl just gets the run off from the tank.

The drain field should not have an effect on the filling of the tank , and really is shouldn't have much of an effect on the draining either, unless you have had a lot of rain and the ground is saturated with water. If you have a septic tank it might need to be pumped out, especially if you have had a lot of wet weather, usually you can get it pumped out and the toilet will work better.

If the toilet is filling slower, then the fill valve inside of the tank could be let go from the fill tube a little.  Look inside the tank to see that all of the water is going into the fill tube.  Flushing slow has to do with the place that the waste is going.  Too full of a tank or not enough vent for air to go.