Definitely Kopia. I was skeptical at first, it's a relatively new tool and not much is known about it (at least on HN) but it's uber-fast and compresses extremely well. Detection of duplicate blocks seems both more aggressive and faster compared to Borg and Restic.

It's honestly a win on all fronts. It can sync to remote places -- which I don't care about as I have a local directory where it does its stuff and I then distribute that with `rclone` to several encrypted cloud accounts -- BUT many people do care so to them it's an even bigger win.

So yeah, I tried Borg and Restic and Kopia and Kopia wins by all accounts: size of backup directory and speed of operation at least.

(One thing I should mention is that Kopia is also very aggressive at eliminating duplicate snapshots and sometimes you might find yourself lacking snapshots for e.g. five days as it happened with me; however, upon very close inspection -- think unpacking the two snapshots before and after that period, and checking diffs, it turned out that I lost nothing, everything I wanted was still there. But, it's a disclaimer one way or another. Plus, its policy for retaining stuff is not like that of e.g. Borg. Make sure to read up a bit on it before using. Again, it never loses data but it has a different opinion on what is "the last 10 hourly snapshots" compared to Borg.)