Whenever baloo is doing a completely new indexing, it takes over all the resources of the system CPU, memory, disk. It can render the machine unusable for a few minutes up to an hour depending on the system performance (ssd or not, cpu etc).
There should be a limit to the resources it can use. Even terminal cannot be used during the takeover. Is there any patch to this hostile behaviour?

I just updated to fedora 29, with the same problem. I found out that it was caused by content indexing being enabled by default. In my case, probably due to the huge amounts of postscript and pdf files I have. I had an index file of 12 GB, and the indexing took for ever because the system was forced into swap. After I disabled content indexing the problem went away.