When performing an "undo" in the retouching tab it causes the whole chain of retouching strokes to be performed in sequence (minus the last one). This is quite slow and causes a lot of delay when processing an image.
I assume that it is easy to cache the most recent (before the current) result in memory, such that a single undo (the most common undo operation) is fast. Going back further if necessary can then recall the whole processing stack.
If you can implement this it will really make a big practical difference to me!