I think what's happening is when you have a system that has not been fully indexed, then when you are not using much CPU power, it uses the spare power to index the system. So yes it will run hot while it's doing a lot of background work to index the system.
The more you use the computer, the less it has to index (because it's already been indexed). And once indexing is essentially done and only new things have to be indexed, the "running hot" should occur less frequently. This runs counter to how a lot of people use a computer. Some people boot up, do some tasks, and shut down. Which means there is limited time available for a computer to index and do maintenance.
Indexing is important for search. If you want to look for a document that contains the word "extraneous" and you type "extraneous" into the search box, a fully indexed system will pull up the document pretty quickly. If your documents haven't been scanned and indexed yet, it will either take a much longer time to find it or it will not find it at all.
Yes, killing tasks will improve short-term performance. But that's like telling the librarian to stop putting books on the shelf.
However, some of these background tasks can run away at times due to glitches. Rebooting the system and/or clearing caches and rebuilding the index helps with getting things back on track.