Going back now I see that time is actually flowing in the expected way (past to present) but going right to left (not the expected way, at least in western countries) which might explain why I was so confused.
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.