Keep Kindle WiFi 3 plugged in for indexiing large amounts
When I downloaded the Unabridged Matthew Henry Commentary with DVJ by Osnova, I kept it plugged in overnight. That will insure enough current to complete the job.
Otherwise, if done with inadequate power, it may require several sessions of incomplete indexing and never really finish.
That Commentary was 27.7 Mb versus the average book of 1.5 Mb.