Flapper closes to slow and the flushe is run's more water than before.

It sounds like your need to adjust the chain, here is what you need to do.
Feel around the flapper flat edge. If it's wavy or you feel rough spots replace the flapper,The linkage between the flapper and the flush lever should have 1/4" play when the tank is filled. Also, check the small tube coming out of the top of the ballcock and make sure it's connected to the white overflow tube and draining water in to the tube, good luck.

John

I have the same problem with a new flapper. After the install (including the new chain), it just empties ALL the water instead of 1/2 - 2/3 of the tank. I tried all the 5 chain links what could count at all to set the length. The problem is that the flapper opens up and stays for a while, then goes to a higher, full open position and it stays up there until all the water is gone and gravity flips it back down.
I also tried this with a foam in the middle / lower part of the chain but there was no real difference - except the foam can stuck under the flapper if the foam is too high on the chain.

I would be very thankful for some help here.

The solution was to cut a small hole into the flapper, into the bell shape part, just under the 'hat'. Probably a pencil could almost fit through the hole.
When the flapper is in upright position the air will go out earlier on this hole and water goes in, so the flapper will fall back earlier. You can set how early with a piece of foam on the chain. In this way about 30% less water is used for a flush almost all the time but the rest in the tank is still available if needed.