Indeed, I forgot to detail the server hardware configuration, I’m sorry for that.
It’s an “HP ProLiant MicroServer Gen8” updated with an “Intel Xeon E3-1265L V2” processor (passmark 5022), “2 x 8 gb ECC 1600 mhz in dual channel” and “4 x Western Digital Red 4 To” in Synology Hybrid RAID, ext4.
It’s actually running Xpenology in BareMetal (it was my first NAS build a few years ago, today it’s a choice I regret and would like to switch to Unraid instead).
On the other hand, even if it’s not a very high-end NAS configuration, it works pretty well from what I’ve seen (apart from the problem mentioned).
On other users (even a “managed” user on the same client), it loads quickly and is capable of transcoding 4k SDR without a hitch.
Just for curiosity’s sake, what passmarks score would be acceptable?
Yes, I’m comfortable with the command line/ssh, I’m a developer.
I’ve already repaired the database several times following this guide: https://support.plex.tv/articles/repair-a-corrupted-database
Unfortunately, this did not solve the problem of slow library loading for the admin user.
If PlexDBRepair can do more or go further than the Repair a Corrupted Database guide, I’m willing to give it a try.
Anyway, thanks for your time in helping me, I really appreciate it 
