Yes, the timelines are a bit confusing, the one on top is a zoomed in version of the one below. The one on top also shows also shows the time between the edits.

To show the change happening the tool fetches the html of every edit. So for a long article with a lot of edits it takes time to load the edits & to start the playback. There is a small slider that lets you control the speed of the playback.