Hello,
I am aware that kde-baloo.service already sets IOWeight=1.
What I see monitoring the indexer, though, is that the indexing of new files starts immediately as new files are added to the included folders. Even before the file is complete, in the case when no temporary file (i.e. .tmp) is used.
It seems to me that it would make sense to be able to delay the indexing of a new file by a few seconds, so not to write the new file at the same time Baloo is writing in the index.
Let me know if I am mistaken in my understanding.