How to speed up slow zpool scrub?

[Miroslav Lachman]

Hi,

is there any way to make zpool scrub faster?
We have one older machine with CPU Pentium(R) Dual E2160 @1.80GHz, 5GB 
of RAM and 4x 4TB HDDs. It is just a storage for backups for about 20 
machines.
Scrub is scheduled from periodic each 30 days but it takes about 4 days 
to complete and everything during scrub is slow. Backups takes 8 hours 
instead of 5 (made by rsync), deleting of old files is even more slower.

The backups are made every night from the midnight to morning, the 
machine is idle for the rest of the day.

Is there any tuning to make scrub faster in this idle time?
Or is it better to do it other way - slower scrub with even lower 
priority taking for about one week but not affecting time of normal 
operations? (is it dangerous to have scrub running this long or reboot 
machine during the scrub?)

I have a performance graphs of this machine and CPU is about 70% idle 
during scrub, but hard drives are busy 75% (according to iostat)

FreeBSD 10.3-RELEASE amd64 GENERIC

Miroslav Lachman