TL;DR: GIMP 3 is out!
It means that 3.0.0 is released. A git tag has also been made. Not sure why a release hasn't been added to GitLab yet, as it has already been 8 hours.
In practice, this means that GIMP version 3.0.0 (actual release, not a release candidate) has been released and can be built.
I haven't used either, much less benchmarked them, but the performance differences should be negligible due to the IO-bound nature of the work. Even with compression & encryption, it's likely that either the language is fast enough or that it's implemented in a fast language.
Still, I wouldn't call the choice of language insignificant. IIRC, Go is strongly typed while Python isn't. Even if type errors are rare, I would rather trust software written to be immune to them. (Same with memory safety, but both languages use garbage collection, so it's not really relevant in this case.)
Of course, I could be wrong. Maybe one of the tools cannot fully utilize the network or disk. Perhaps one of them uses multithreaded compression while the other doesn't. Architectual decisions made early on could also cause performance problems. I'd just rather not assume any noticeable performance differences caused by the programming language used in this case.
Sorry for the rant, this ended up being a little longer than I expected.
Also, Rust rewrite when? :P