This morning I noticed that the fan was running fast on my laptop, getting rid of cpu heat. It rarely does this, and the last time I found a cause it was a bug in Windows that I found a workaround for.
I had a thought, and closed two very old tabs in Firefox that were showing OGS puzzle pages (tsumego), just keeping my place. The fan immediately became quiet.
Then I opened OGS again to send this report, and the fan started up again. Is it possible that simply connecting to OGS is enough to keep the cpu very busy?
It matters because (1) hot cpus have, I believe, a shorter lifespan, and (2) it will shorten battery time when running on battery power.
Note that this could also have been due to a tab containing a running video, which could have coincidentally reached the end of the video at the same time that I closed the tsumego tabs. But it is worth asking an expert to find out whether keeping a couple of tabs with tsumego game boards open loads the cpu.