Thanks everybody, everything seems reasonable for me too now.
Why having an extra layer of security if it is not needed?
In my case, my own data and stuff isn't worth encrypting, but when I'm helping others, I take all the precautions of not leaking
their data, regardless of whether they consider it worth encrypting or not.
If I keep any backups of others' data during helping – I delete the backups
always afterwards –, I do it on physically secured external storage.
And yes even if the performance losses are small, they are just irritating if not necessary.
In my case, even on a fast SSD, it is neglible. That is, I cannot tell whether the data I'm accessing is encrypted or not.
My own backups are mostly source code and emails, and I don't encrypt those. (Any emails with proprietary information are moved to the abovementioned external storage and deleted from my mailbox.)
Why so "paranoid", one might ask? Because it is a very easy way to completely ensure I'm not the weakest link, security wise. Nowadays it is second nature, and not a bother at all. When traveling, I always use a secondary laptop (one I can afford to lose) with a pre-prepared, initially completely wiped one, that contains only the minimal amount of information I'll need. This also means that if e.g. border control or customs wants full access to it, I have no problem with that.
I also keep my passwords in plaintext in a 2D array of characters, where the password is read in a certain orientation (not in a straight line in any specific direction or starting from a specific position). The width and height can even be smaller than the length of the password. While this is not "secure" in the way encryption is secure, it is sufficient to stop any casual intrusion. Anyone with the werewithal or resources, can always just put a gun to my head and force me to decrypt the data, so there is no such thing as absolute security.
On servers, my preferred security measures step up a couple of notches, though, but that's a separate topic. I'll just mention that there, leaving an immutable audit log of privileged accesses (traceable to a human account) is paramount. Missteps, excesses and intrusions will happen because we're human, and when they do, an immutable audit log is often the only thing to find out what happened, and to deal appropriately with the people involved. (Many CxO's hate this, because it makes it impossible to throw a scapegoat underling under the bus when they themselves err.)