Description of problem:
mlocate's cron /etc/cron.daily/mlocate.cron sets nice-level to 19, but I/O priority only to best-effort with low priority 

The latter is pointless as that best-effort-low-priority is already accomplished by setting the nice level - io_priority = (cpu_nice + 20) / 5.

Furthermore the best-effort class doesn't really have that much of an impact. It should better use the IDLE class:

#!/bin/sh
nodevs=$(< /proc/filesystems awk '$1 == "nodev" && $2 != "rootfs" { print $2 }')
renice +19 -p $$ >/dev/null 2>&1
ionice -c3 -p $$ >/dev/null 2>&1
/usr/bin/updatedb -f "$nodevs"