You probably already know you can hit the j key, in the render view, to flip to another slot. You can render with that slot revealed, and when the render is done, you can hit j repeatedly to toggle back and forth, comparing the new render to the previous render.
But j only steps forward. When you add 3 or more slots, it starts getting tedious.
Today I learned that alt j will flip backwards. So, if I have a rendered image in slot 2, and another in 3, I can go back and forth without seeing slot 1 again.
Nice! Wish I had noticed that 10 years ago.