Interestingly, the changelogs are silent about a fairly major change to the filesystem when going from 10.3.x to 10.3.3. It seems like APFS was originally intended to use a different unicode normalization setup than HFS, but it turned out to be very problematic. After iOS 10.3.0 silently converted all iOS devices from HFS to APFS (!) (and not only was this not specified in the user-visible changelogs, earlier iOS 10.x.x releases did the same dry-run conversion without notice - only stopping short of committing the final type flip - which may explain why iOS OTA upgrades have been somewhat slow to execute -
https://www.macobserver.com/analysis/apple-dry-run-apfs-prio... ), iOS 10.3.3 adds runtime normalization to the file system. It's unclear what kind of performance hit this has (but I seem to remember reading something about how samba on UNIX taking a hit on file opens in order to support clients specifying incorrect casing, which sounds similar). Apparently an unspecified later version of iOS will perform yet another conversion, from APFS-normalization-preserving to APFS-native-normalization.
More details: https://mjtsai.com/blog/2017/06/27/apfs-native-normalization...
By the way, if you ever rsync between macOS and Linux you may have noticed (or not) how this unicode normalization messes up filenames and cause duplicates and stale copies when roundtripping, see https://serverfault.com/questions/397420/converting-utf-8-nf...
..
Also, unrelated, it seems this version of iOS fixes the Broadpwn wifi chip vulnerability (which perhaps could also continue on to compromise the main OS kernel via a DMA attack after compromising the wifi chip) ( http://boosterok.com/blog/broadpwn2/ , https://nvd.nist.gov/vuln/detail/CVE-2017-9417 )
More details: https://mjtsai.com/blog/2017/06/27/apfs-native-normalization...
By the way, if you ever rsync between macOS and Linux you may have noticed (or not) how this unicode normalization messes up filenames and cause duplicates and stale copies when roundtripping, see https://serverfault.com/questions/397420/converting-utf-8-nf...
..
Also, unrelated, it seems this version of iOS fixes the Broadpwn wifi chip vulnerability (which perhaps could also continue on to compromise the main OS kernel via a DMA attack after compromising the wifi chip) ( http://boosterok.com/blog/broadpwn2/ , https://nvd.nist.gov/vuln/detail/CVE-2017-9417 )