I'm seeing this behavior as well. For a test keyring with 13 keys, 49 User
IDs, and 7227 signatures, stored on a tmpfs on linux kernel 3.16, gpg2
--list-sigs with a pubring.kbx takes over 3 seconds (mostly kernelspace), but
with an old-style pubring.gpg takes 0.6 seconds (mostly userspace)
Maybe it makes more sense to mmap the keybox rather than trying to seek and read
inside it?