Weren't thumbnails supposted to "carry through" Backup-Restore from one PC to another, and avoid the time consuming rebuilding?
No. Thumbnails aren't included in MC backups. It would make the backups too large.
There have been some requests around this topic, and carrying them over from previous major versions during an upgrade. That led to this change.
26.0.7 (11/21/2019)
8. NEW: Thumbnails are copied on an upgrade install.
The portable install may differ from a PC install, in where the thumbnails are stored. If the thumbnail database is in exactly the same location, MC may use it. On normal PC installs using default locations for a new major version, the thumbnails database is stored in a new location, so the old database is not used. Hence the change above to copy them (actually, probably only the one thumbnails database for the default Library) across.
Regardless, if the thumbnail database became "dirty", where the indexing didn't match files after an upgrade, MC would rebuild the thumbnails.
Slow thumbnail rebuilding has been addressed in the past, but in your case it may just be because you are using portable installs. Search the forum on the topic and see what you can find. The stopping and restarting the build to improve speed was fixed at some stage, I believe.