More thoughts on this:
Several days' more experimentation have shown that if I leave my Kindle along, connected to a power supply, for anything up to about 20m, once it goes to sleep (which happens after about 5m of inactivity) it continues to index at full speed. Leave it for longer than about 20m, though, and indexing essentially stops, only resuming again when I wake the Kindle up by opening the cover or pressing the power button.
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