Upgrade v7 -> v8 Repository Sync Duration

[AlexL]

We have a number of larger customers that are still running on v7 but we are finally starting to upgrade them. The upgrades so far went fairly smoothly.
The upgrade of the object repositories seems to be the part that takes the longest time, the install/upgrade itself is fairly simple and straightforward.

What would be the most determining factor in the repository upgrade time?

Is it number of items (mails/files etc), or the number of user accounts (or objects) or is the size in TB of the data?

For example, how would a customer with 3600 users and 138TB of data compare with a customer with 8000 accounts but only 40TB of data.
The former was done within an hour (2 repositories), the latter will be upgraded soon.
Previously I've upgraded another customer with 1200 users and 12TB of data in 1 repository, that was done within 10 minutes or so.

These three customers are the smallest of our larger MS365 backup customers.

Furthermore, the repo upgrade and following sync seem to be both compute heavy and disk io intensive. Would it be wiser to upgrade one repo at a time or multiple, or even all, at the same time? Considering the goal would be to minimize the total upgrade time, and as such down time, for all repositories.