Hey! So my intuition is that somewhere there are temporary files saved. It could be anything from all files that are used by the program or it could be only stuff like icons. I think that anytime the program loads up, it generates a set of files, saves them to a folder and loads everything that is in that folder. I think this would happen somewhere the operating system cleans up after a restart. Something like a cache dump place for quick access of recent files. I don't have any other possible explanation for this, as a memory leak would simply cause more and more stuff to be loaded into memory after a while. I can even imagine it being some file with a list of assets. And each time you run it, it simply adds more entries to the list instead of generating the list again. And then in the main function of the program, it checks the list at every tick. With the list getting longer every time the program restarts, that could seriously slow down the entire process. That's something a super heavy PC with little assets added to the program may not experience for a long time or not at  all. But someone who adds as many of the kenney packs to the Asset forge as possible and then starts adding their own may end up getting there much faster.

Also, I found that logging out of your account on your PC and logging back in works just as well as a full reboot, and takes much less time. But I seriously hope we get some sort of fix for this in a future update.