$ time gpg2 --homedir=/var/tmp/gh -u 16add702 -v --clearsign aa.txt
gpg: writing to `aa.txt.asc'
gpg: RSA/SHA256 signature from: "16ADD702 Niibe Yutaka <gniibe@computer.org>"

real    0m8.733s
user    0m0.020s
sys 0m0.008s

It's in the rsa4096support branch: http://git.gniibe.org/gitweb/?p=gnuk/gnuk.git;a=shortlog;h=refs/heads/rsa_4096_support