The closer you get to lapping the stone flat the longer it takes to lap the higher areas down. This is because you have a flat area that gets larger as you remove material. It can take forever to reduce the whole surface enough to eliminate a rounded corner (as you know). I wouldn't worry about the corners. After you radius the long edges of the stone you will have a honing surface that is very slightly curved inward at the ends where you don't use it anyway.