The "revisions" are from a plugin themselves (my Edit History plugin). You can delete revisions manually, but I'll look at adding a way to delete multiple revisions. Maybe it would make more sense to delete SOME revisions but not all.
If you want to clear ALL revisions the easiest method currently is to truncate the qa_edit_history table e.g. with phpMyAdmin.
I don't think clearing the table will make a huge difference to speed, as when viewing questions it only does a simple check if there are revisions (it doesn't load all the revisions). But you could set the visibility in plugin options to a higher user level like Moderators. That way for most users, viewing questions will not touch the revisions table at all.