Hi. I'm sorry that you consider this a bad experience but please understand that your context is way outside of a normal user situation. The addon just tries to save the data into storage and I have no way of configuring how and where that storage goes, it's what the API provides. As regarding the configuration, you have the option to not use the saving at all (disable) or you have the option to change the interval of the saves, there is not really anything left to do, and the timing was set to 15 seconds because most users have a few tabs opened and saving them will not require substantial IO or storage. In your case yes it does not makes sense but that's why you have the options to change. The storage space is just made up of the URL and title of the addon and the icon of the site nothing more, so there's no real overhead that I create other than what it is required for a session.

Please also not that the addon description clearly states that "THIS ADDON IS NOT INTENDED FOR AUTO-SAVING OF SESSIONS!".
Having this in mind, giving a 1 star rating to something that is not intended for your own purposes and in that context of having hundreds of tabs it's a little bit excessive.