is it really worth using encrypted backup software to backup my data, when the cloud storage is already E2EE?
It’s always the user’s choice, of course. But it’s worth knowing that double encryption is standard for government-sensitive data — it’s mandatory policy, since we can never be sure that any particular encryption method has no bugs. This is so common that when cryptographers design new software, they assume their app will be one layer of encryption, not the only one:
Saying this I don’t mean everyone should do this. One encryption layer is fine, no encryption is fine too in a lot of cases.
But are you sure encryption is what’s making the backup slow? I used Kopia for a while and remember the devs were skeptical about non-S3 backends with rclone — and using Kopia with Filen you’re actually using rclone, i guess. Filen is also historically not known for high speed, though I haven’t tested it for heavy uploads after the recent update that was supposed to fix speed issues.
Could you first test your regular Filen speed as well as uploading a Kopia backup to S3, to see where the real bottleneck is? I’d guess it’s not the encryption.
Finally, from my experience living with slow wifi while traveling: I ended up making regular remote backups of just the small part of my data that’s most important or changes often, and sticking to full backups for everything else, triggered when visiting an office or library. Maybe you don’t need the same backup policy for all 350GB?