.pack vs zst-7z with the same compression settings would b interesting. That will be pure container overhead
Portability: Receiver (or future you) needs to know what you used, and what version even.
Speed: If you want to do the archive part first (tar) and then compress (gz), you will get much lower speed (as shown in the note).
Reliability: Most people use tar with gz anyway, but if you use it with not so popular algorithm and tools, you will risk having a file that may or may not work into the future.
Pack plan is to use the best of time (Zstandard) and if an update is needed in years to come, it will add support for the new algorithm updates. All Pack clients must only write the latest version (and read all previous versions) and that makes sure almost all use the best of their time.
Yes it is that much faster, and a good part of it is because of the multi-thread design, but as a reminder, WinRAR or 7-Zip are too multi-thread, and you can see the difference. To satisfy your doubt, I suggest running Pack for yourself. I am looking for more data on its behaviour on different machines and data.
Can I ask why do you need a version without ZSTD? If you are thinking that compression slows it down, I should say no. Pack is the first of its kinds that "Store" is slowing it down. Because its compression is smart, it will skip any non-compressible content.
On the same machine and the same Linux source code test:
Pack: 194 MB, 1.3 s
Pack (With no Press): 1.25 GB, 1.8 s