samba-tool ntacl sysvolreset is slow due to the internal operations done to confirm if a user is a member of a group. This operation is repeated multiple times for each file, and this makes the tool very slow on large sets of GPOs.
Partially fixed in Samba 4.9 by caching the user's groups. There are still issues but this can be closed as fixed in Samba 4.9 by a8a9bb553c8195425f385eee1cc8efab8cd889e9