I’m keen for a third toggle button to be added next to the off/on switch at the top of each pallete. Its purpose would be to change the state while the mouse is pushed over the new button. I see the changes due to a specific adjustment now by now by flicking off/on/off/on, but having a little button next to the existing off/on switch would make things so much easier and encourage that type of preview.
I’d suggest that click and hold would preview, while short click would toggle. Most people use short-click so the longer hold preview wouldn’t interfere with the current interface.
So after click and hold, the changes would revert and the toggle would not move. After short click the toggle would flip and the changes would show up as fast as Photolab can process them.
I would find preview without toggle useful as well but it’s not all that important to me. For most palettes, I obtain a similar instant preview by dragging the primary slider up and down.