I must say that is quite an insane amount of saves, no offense
Even i have sat around 120 or so as maximum through all days of development.
The reason why it takes so long is because of building that big ass save list and indeed the thumbnails do cause a performance hit. It doesnt matter so much if it's webp or any other format in this case because it all gets loaded as a texture in runtime, and displays that texture as a thumbnail.
However, some good news for you is that i have been working on some save/load menu improvements, you will be able to organize your save files either in your system through the file manager, into folders, or through the game, create and delete categories (which are regular folders in your filesystem).
I have to ask though..... why so many saves? Pretty much 99% of the time where there is a choice the game automatically does a micro save inside your save file which allows you to go back to a previous choice. So you at least dont have to save at every choice like in many other engines, for example ren'py where i too save like crazy. Then ofc we also have the replay menu which allows you to replay pretty much if not all (unless i missed something) sexy scenes. It doesnt have all the non-sex scenes in it though.
You could probably either clean up a little or temporarily archive some of your saves in subfolders of the save folder, if you dont need them anytime soon
/A