Adding encryption to Btrfs

I'm not aware of any published comparisons. I've done some informal testing on my own machine, a quad core Skylake running at 4.5GHz, with 8K blocks and found:

ChaCha20 achieves 16.4GB/s while consuming 102W or 5.93 microjoules/byte
AES-128-CTR achieves 22.4 GB/s while consuming 87W or 3.70 microjoules/byte
AES-256-CTR achieves 16.6 GB/s while consuming 82W or 4.71 microjoules/byte

You might argue comparing AES-128 to ChaCha20 is unfair, but the fact is those are by far the most widely used variants of each.

ChaCha20 was tested using the benchmark tool from https://github.com/floodyberry/chacha-opt modified to run ChaCha20-avx2 with 8K blocks in a loop
AES was tested using the example code from https://wiki.openssl.org/index.php/EVP_Symmetric_Encrypti... modified to encrypt 8K blocks in a loop.
All tests were done with 4 instances running in parallel.
Power consumption was measured using CPUID HWMonitor.