Slow Time Machine backups and Chrome browser

This isn't a question, but rather a tip. My Time Machine incremental backups, which usually take less than a minute suddenly become very slow. I selected "Open Time Machine Settings..." to view what was happening. In the text "Found nnn changes", the number nnn was rising very very slowly. I closed Chrome, and the nnn count suddenly speeded up dramatically. My backup then finished in a few seconds.

More info: I had two Chrome windows open, one of which had about 20 tabs open (all in YouTube), the other about 10 tabs.