Longest. Progress Bar. Ever?

I've been using a MacBook Pro for a few years, which I’ve been quite happy with except for some annoying behavior when resuming from hibernation. Being Security Guy by habit, I’ve unsurprisingly been using FileVault disc encryption; after typing a password to unlock the volume, I’d often been waiting for a minute or so, sometimes spanning a screen blank and reset button tap, before being presented with another password entry prompt and the chance to enter a useful system. (Deceptive UI indication aside: please don’t provide a blinking cursor in a text box before it’s ready to accept input!) Accumulated impatience having eventually spurred me into action, a web crawl yielded references like this article, suggesting that OS X Yosemite was intolerant of at least some earlier FileVault configurations and recommending decryption and reencryption. I started the decryption on a disk with about 140 Gb in use; though it changed before I got to screenshot the display, the progress bar started out with a caption estimating 43 days to complete the task. That proved conservatively pessimistic, with the operation taking about 10 hours in the actual event. I’m used to symmetric crypto being quick, often invisible inline at human scale; 43 days read more like what I’d expect if a fairly substantial exhaustive search had been required.