Mac – Double encryption when using encrypted Time Machine on MacOS Extended encrypted

Make sure that your partition scheme is GUID, not Master Boot Record (MBR) or Apple Partition Map.

To enable encryption, the partition map must be changed to GUID. Note that this will make your drive invisible to older PCs and will wipe all of the data on the drive.

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.