I started some experimental multithreading code there:

https://github.com/thomaswaldmann/borg/tree/multithreading

especially:

ThomasWaldmann@240a27a

From Python c-api docs:
"the standard zlib and hashlib modules release the GIL when compressing or hashing data."
So current compression and hashing should be ok for good multithreading, as well as python's I/O (it releases GIL before I/O ops).

Additionally, later changesets implement code to release the GIL there:

• chunker
• crypto
• lz4 compression

💰 there is a bounty for this