@kbm0 Three thanks to you: one from me personally, one from my left eyeball, one from my right. I had found some pieces of the puzzle previously, but probably wouldn't have realised by myself that I need the old Liberation instead of the new one.

@deshipu The numbers 12309 have been burned into my mind a long time ago. https://bugzilla.kernel.org/show_bug.cgi?id=12309 says "CLOSED CODE_FIX" but that's evidently not the case.

@trogluur The correct answer is always "it depends" (and "profile it!"), but it's quite plausible. More than a decade ago it was all the rage to install ureadahead (so that the files would be already buffered when you need them) and prelink your system (to spend less I/O resolving dynamically loaded shared objects). Though with modern storage, the size difference would have to be quite drastic to make a noticeable improvement.

@ArchusByte Take a look at man 5 apt_preferences. It should be possible to match your non-Mint repo by some keywords, at least origin, and exceed the priority of the Mint package.