If you have several busy JFS filesystems all using one log volume, there is contention for the JFS log; which slows down all the filesystems. You can either add another log to some of the filesystems, or switch to JFS2, which can put the log inside each filesystem. To add another log:
mklv -t jfslog -y new_loglv_name vgname 1 logform /dev/new_loglv_name
After that, edit the entry for the filesystem in /etc/filesystems, and update the “log” parameter. Then unmount and remount the filesystem.