zeopack taking very long to finalize after replacing old storage

[zopyx]

ZODB 5.5.1, Python 3.7.3

I packed a filestorage with 30 GB size down to 11 GB. After replacing the Data.fs with the packed version it took about 30 further minutes until zeopack actually finished. What's happening after replacing the Data.fs? Is this due to writing a new index or just Python garbage collection taking place?

Local resource issues are unlikely the problem (decent CPU, enough RM, SSD, no swapping)

[zopyx]

Likely related to generating the index file (created 32 minutes after replacing the Data.fs).

s -la  var/filestorage/
total 41311940
drwxr-xr-x 2 ajung users        4096 Jul  1 10:20 .
drwxr-xr-x 6 ajung users        4096 Jun 29 07:58 ..
-rw-r--r-- 1 ajung users 11540432977 Jul  1 09:48 Data.fs
-rw-r--r-- 1 ajung users    22226481 Jul  1 10:20 Data.fs.index
-rw-r--r-- 1 ajung users           7 Jun 29 07:58 Data.fs.lock
-rw-r--r-- 1 ajung users 30726274928 Jul  1 08:42 Data.fs.old
-rw-r--r-- 1 ajung users    14460445 Jul  1 08:29 Data.fs.tmp

The site contains one fresh Plone installation (result of a full export-import migration) with about 100.000 content objects and 25.000 blobs..The blob storage is mounted on NFS..perhaps that might be the issue?