The speed you are talking about, "80/90 MB/sec", is much slower than it could be if you didn't have those 12 disk laid out as a single large RAID-z2 vdev. I have 12 disks in both of my systems and laid them out as two vdevs for the purpose of greater speed. On my 10GB network they will transfer (large files) around 550MB/s and they completely max the wire speed on the 1GB network. Many small files slow everything down, even when you have high capacity for sequential transfer, small files kill the performance. So, the thing that may be causing you more trouble than you realize is the kind of file access you are dealing with. I know you said it is a big file, but you also said the goal is to search for data inside the file and that changes things because it isn't like just moving one big file, which would be fast, it is more the kind of work of accessing many small files.
Like @Stux said above, you might need to put the data for this on SSD or a bunch of mirrors like I suggested to get reasonable performance.
Also a SLOG device might help. Your current hardware is just not going to do it, not without making some changes.
Thanks. Appreciate the detail.
One last thing came to mind.. A year ago, I set my TCP control algorithm to 'cubic' to handle better wifi speeds. Is there a chance that this is messing up my iSCSI? ( https://forums.freenas.org/index.ph...very-slow-only-to-freenas-sanity-check.43811/ )