When you add mirroring, your file-system will generate more overhead. Simply stated, you sacrifice speed for stability. Doesn't matter which file-system and/or RAID setup you use.
Also, verify the health of the disks in your storage pool(s). It is possible that one of your disks is generating a lot of interrupts, dragging the speed of all other drives down.
There is still a long shot (especially with older drives): drive alignment. Any partition management tool (Minitool has freeware for this) can verify this for you and allows you to align one or more partitions. If the partitions id filled with lots of data, this will take quite some time (hours). An empty partition is aligned within seconds. This really matters if disk I/O is of significance to you.
If you want speed, having a trustworthy backup strategy in place for your data and turning off any mirroring functionality is your best bet.
And if you are not comfortable with that, then your backup strategy isn't as trustworthy as you think
