Use action=compare instead. Text to diff each revision to. Only diffs a limited number of revisions. Overrides $2diffto. If $2section is set, only that section will be diffed against this text. For performance reasons, if this option is used, $2limit is enforced to $1.
You shouldn't even be seeing this. Bizzeebeever told me to add this here so that we can get responsiveness working for all screens.