I assume it's just not built to be fast, because it's still slow even with MySQL, Redis, high PHP memory limits, a fast CPU and NVMe storage, and so on.
Last time I tested it I had a load time of 1-2 seconds just to bring up the files interface, it feels laggy no matter what. And syncing a folder with ~50k files and 40GB or so in size takes a very long time compared to Syncthing or just syncing over SMB.
Regardless of what it is, make sure your backups are working, running often (daily or better is good), and test your restore process fully.
Fair enough, it does add a good chunk of power usage though as HDDs are pretty power heavy at 5-7W or so.
Set up a minimal server VM instead? You don't need to install a GUI on a VM.
Debian minimal uses very few resources, that's what I use.
https://github.com/filebrowser/filebrowser might be what you want for that, just a basic web based file management tool.
But you could also just use SMB and access the shares directly from file explorer.
You end up wasting a ton of space though because each vdev has its own parity drives.
Probably, sometimes I feel like some of the linux community thinks stuff made in 2015 is 'recent hardware' lol
You should have been able to boot from USB and install fully in UEFI mode with Secure Boot enabled, but sometimes the method you use to make the bootable USB can screw that up. I've always had good luck with Ventoy.
Plex/Jellyfin is automatically managed by Sonarr/Radarr so I don't touch those.
But for game servers I use Pterodactyl which has a nice WebUI to manage the server and its files, and has automated backups.
I run the quietest fans I can find at reasonable prices in everything, so Arctic F12 mostly. Those San Ace look neat but 50dB is just way too loud for my setup!
None of them, because every manufacturer has made good and bad products. Seagate had really bad 3TB drives which gave them a lot of that reputation.
I just buy whatever fits my budget for HDDs and have proper backups in place. I think almost all of my HDDs are 'refurbished' ones.
For SSDs I look for one with a good TBW rating with a cache in it. Typically I'll go for used enterprise SSDs as well.
They're also useful because they're easy to deploy, contain all the dependencies needed, portable, and isolate things breaking from affecting the host or other containers.