I'm using libgpg-error 1.26, though I'm sure it also happened with 1.25 (I get
libgpg-error from Debian unstable, which went to 1.25 on Nov 16th, and then 1.26
on Dec 24th, and I saw the symptoms at both of those times). I'm happy to
experiment with another version if you have suggestions.

The initial motivation for looking into this is that Git's test suite, which
uses gpg --import to set up some test vectors, was taking longer to run.
There's some additional analysis on the Git list, especially:

http://public-inbox.org/git/20161228160520.dsybuqljzkzsik2d@thunk.org/