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?
Actually your mac is capable of encrypting that fast (perhaps you're confusing it with encryption+compression). 

Your processor has a special Intel AES-NI instruction set, specifically designed to optimize encryption speed. Looking at some performance benchmarks for that processor, the AES test performs at a whopping 8.87 GB/s multi-core and 2.94 GB/s single-core. No bottleneck there, but I can imagine some latency from processing thousands of small files instead of one big one like in the benchmark.

To address the confusion in your question:


While logged in, encryption happens on-the-fly whenever you read/write from anywhere on a FileVault2 encrypted disk.
Time machine transfers data unencrypted to the external disk, then encrypts after. My guess is they designed it this way to be more fault-tolerant with slower finicky external media.