How do you view the diff of a change when you turn this option off?
Normally, when you move between revisions the diff window will show you what change that particular revision did. Example:
A -> B -> C -> D
When you move the cursor to B it'll show you the diff between A and B.
The `p` key shows you a preview of what would happen if you reverted to that state from your current one. If you were at B and pressed `p` it'll show you the diff from D to B, not A to B.
I feel like Gundo becomes kind of useless without seeing the diff so you know what the undo state you're looking at is -- without it you're just looking at nodes that all look the same. I'd much rather improve the performance so it's less of an issue.