No real surprises here
unRAID's MD device array provides very high flexibility in terms of storage options (being able to use any drive size in the array that is smaller than parity), and preserving individual array drives in the case of a failure that exceeds the capability for parity protection, rather than losing all data on the entire array (you'd only lose data on the drive that failed).
The price of all that flexibility is speed.
For many types of array data, that's not a big issue, but occasioanlly for huge numbers of files, it's very slow (especially for some lighter-weight systems).
Using a ZFS striped array in unRAID (same as TrueNAS) should provide the speed you're looking for, but you lose that amazing ability to just add drives and expand/change your storage needs almost effortlessly.
The great thing is that you can have your cake and eat it too!
My array has 13 unRAID-style array drives with dual parity, protecting all of my media (who cares if it's a bit slow, just TV shows, Movies and Music).
All of my important stuff (bank documents, kids stuff, family photos, etc) is on a much smaller ZFS array that's replicated hourly with snapshots to a second independant unRAID server, and from there, weekly to a tiny 2-drive ZFS array that gets rotated-out to a safe deposit box every few months. I used to use rsync to do that, which was slow and painful. With ZFS and snapshots, it's effortless and super-quick.