Well, that explains it...

The following are known bugs in Audacity 1.2.2:

• [...]
• Linux: Recording in full duplex (play existing tracks while recording) on some systems causes mono recordings to sound slowed-down.

Guess I don't need a faster system after all. But recording to a network-mounted filesystem seems to fix the problem.

And it's moot in any case, since I seem to do much better recording voice and guitar at the same time.