Baloo Author here, though I haven't contributed code for over 4 years now. Baloo used to use Xapian under the hood, like Recoll, when initially launched, but i

Baloo Author here, though I haven't contributed code for over 4 years now. Baloo used to use Xapian under the hood, like Recoll, when initially launched, but in 2014 (I think?) I moved away from it to a custom DB built on top of LMDB. It resulted in a massive performance increase, one of the main criteria was for results to always take less than 'x milliseconds' (I can't remember exactly how much) so that KRunner (similar to OSX Spotlight) would always feel snappy. Another criteria was for Baloo to operate well with minimal memory and CPU, which wasn't the case with Recoll (Things might have changed since then) I do remember ensuring that Baloo always did content indexing in another process, though I can't remember at what point it blacklists a file. Sorry, this isn't the best answer to your question, but most of the details are now quite fuzzy to me, and I haven't kept up with how the project has changed after I stopped contributing to KDE. In retrospect though, I definitely should have spent far far more time ensuring that Baloo doesn't end up in situations like the one you described, and putting far more limits on the IO and CPU usage it's allowed to use.