Do you have any tests to compare plain squashfs xz with zstd? The nested ext4 stuff is really pointless now because Fedora hasn't used 'dd' + resizing the ext4 file system as an installation method in a long time (going back to Fedora 18 I think). All of the Live installations use rsync.
The Zstd compression performed worse than XZ in the compression test. On the other hand, 40% lower installation time for Zstd, was documented. Along with the CPU consumption 37% lower.
All installation tests were performed from and to local NVMe storage. Which I consider far from real life scenario.
Fedora QA nightly tests are real and I think it'll make a meaningful impact for both the creation of the ISOs, as well as their consumption, in a lot of cases. Even if it doesn't impact USB installations. I do VM installs on both SSD and NVMe and and it matters there. But also the power consumption of xz I think is relevant whether baremetal or virtual.