When does encryption actually happen in MacOS/OS X encrypted volumes?

I've noticed a sort-of lazy/post encryption strategy in MacOS/OS X with FileVault enabled and in encrypted Time Machine backups, and I'm curious when actual encryption happens. - When I copy videos from unencrypted external USB 3 volumes to my FileVault encrypted volume, my write speeds are 100+MB/s. However, I don't think my macbook (Intel Core i7-4980) is capable of encrypting that fast on-the-fly. - When I complete a time machine backup, a lengthy "encrypting..." step happens, but continues seamlessly after un-mounting/re-mounting. This is more obvious when encryption is happening, but is the data exposed until encryption is complete? **When does the actual encryption happen? Is there a period of time where my files are vulnerable unencrypted?**