This will seem strange - I couldn't believe it myself. I had one tank refill rate at about 90 seconds and the other at about 25 seconds. When I changed out the mechanism inside the tank that did little to nothing other than both filling a couple seconds faster. I then removed the flexible hose from the wall shut-off valve to the tank bottom fitting. Inside the hose was a device placed there purposely by the previous owner (I'm guessing he was the one from the other weird stuff he did) which looked like it was maybe a flow-restrictor for either rate or pressure, not sure what the purpose was. When I removed that device, the fill rate of the slow tank changed to the same rate as the faster tank. So-o-o-o... Changing the tank mechanism might be a good idea (change out the flapper valve at the same time) and change the flexible line from the wall to the tank bottom connector. You might not have a restrictor like I did but your line may be crimped inside or clogged with debris. The only other item would be the wall shut-off valve but that is not likely the issue. Good luck.