It's been a few days, but I still had the original files in their original locations. So:

  root@darkstar:/usr# date -u; time gzip < local-revert.tar > local-revert.tar.gz
  Tue Oct 11 02:49:32 UTC 2022

  real    24m35.214s
  user    23m57.449s
  sys     0m27.327s
  root@darkstar:/usr# 

From the linked article, for convenient reference, making the .tgz file with tar cvzf took about 28.2 minutes. Making an uncompressed .tar file took about 1.2 minutes. Just now, compressing with gzip took 24.6 minutes. It seems, in this situation, about 2.4 minutes might be saved by creating the tar file first and then compressing separately.

Would be curious about OS / input/output size / hardware/virtualization tech, the only reason I can think for this would be tiny buffers with exorbitantly expensive context switches like you'd see on some older virtualization or e.g. puny escaped-a-VCR ARM chips