@nekohayo A larger amount of text documents would come to mind, or the setting that controls how much text is indexed was changed from its defaults. Tracker checks to vacuum its database during shutdown.
@nekohayo Overall, FSindex seems a nice project, if it does what you want it to do. Looking at a distance, I think adding more features is likely to involve sacrifices to some or several of the advantages you brought up. I bid them welcome to the ungrateful world of filesystem indexers.
@nekohayo 4/ Query speed: FSearch claims as-you-type search speeds, Tracker can make similar claims through full-text search. Nautilus does run in parallel a slow recursive basename search over the filesystem, I wonder if you are mistaking the whole for Tracker.