sysctl -a is slow

just typed sysctl -a on keyboard, and found it is slow, sometimes
it has been stuck for a few seconds, further studied,I found it is
stucked at sysctl kern.geom:

%/usr/bin/time sysctl -a kern.geom
kern.geom.collectstats: 1
kern.geom.debugflags: 0
kern.geom.label.debug: 0
kern.geom.label.ext2fs.enable: 1
kern.geom.label.iso9660.enable: 1
kern.geom.label.msdosfs.enable: 1
kern.geom.label.ntfs.enable: 1
kern.geom.label.reiserfs.enable: 1
kern.geom.label.ufs.enable: 1
kern.geom.label.ufsid.enable: 1
kern.geom.label.gptid.enable: 1
kern.geom.label.gpt.enable: 1
        2.01 real         0.00 user         0.00 sys

it seems it needs more than 2 seconds to complete.