The performance of create snapshot on large MOB table reasonably slow because there are two unnecessary calls to namenode on each Hfile, this while we create snapshot manifest. The first namenode call for getting StoreFile modification time link which used for metrics and another namenode call for getting StoreFile size link which used in snapshot manifest. Both calls can be avoided and this info can be fetched from existing FileStatus link.
PFA. 2x performance is seen after reusing existing FileStatus.