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