Why rsync or copy operations hang, stall, or are extremely slow over NFSv4 due to nfsidmap domain mismatch ?

Issue

• The rsync or copy commands hang, stall, or experience extreme performance degradation when transferring a large volume of files over an NFSv4 mount.

• The /var/log/messages on the client system is inundated with thousands of nfsidmap mapping errors.

   nfsidmap[123456]: nss_getpwnam: name 'user@example.com' does not map into domain 'localdomain'
   nfsidmap[123457]: nss_name_to_gid: name 'user@example.com' does not map into domain 'localdomain'
  

• Transferred files may be mapped incorrectly as nobody:nobody.