File indexing allows (faster) searching of files and sometimes even allows finding files by their content not only the file name.
That said those indexers (I’m not using KDE so not sure about details of your specific case) can use an big amount of RAM especially if there are new files to index. As far as I unterstand they should pause if the RAM is needed otherwise but might depend on the implementation.


Some design choices of Wayland are really weird.
That said do you know of the wlr foreign toplevel management protocol? A lot of the smaller compositors support it. But yes a lot like e.g. Mutter and I think kwin do not.