I recently did a round of 'charge all the Kindles', and I can concur on your observation on charging times when starting from a flat battery
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If possible, waiting for the framework to reboot, but even before that, just a hard-reboot and re-tripping the "low battery" check helps.
I can also confirm that my new motherboard deals with some of those crappy USB controllers much, much better than my old desktop. I haven't had one downgraded to low-speed like I consistently used to
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