When I first setup my unRAID server I used Beta-12.  With a cache drive I was getting transfers to the system in the 100MB/s range, slower than when I used an Ubuntu server with RAID-6, but close enough.  I waited to upgrade until beta 15 or 16, and ever since then when I transfer files of 1GB or so the files transfer at around 24MB/s, same with transfers of large numbers of small files.  And they are definitely going to the cache as I can browse the cache and see them.  Now when I do larger transfers, 10GB or more, speeds seem better, and I occasionally see 100MB/s.  But still, I don't understand why these are so painfully slow.  Is there some setting that I have to tweak now to get decent cache speeds?  I'm running a server with a quad-core i3, 16GB, and a 512GB SSD for the cache, and yet it pales in comparison to my much older AMD based all hard drive RAID system that I used to use.  With the speeds I'm getting there's no point in using a cache drive at all (not that mdadm needs a cache to get 4x the speed). 

Are you sure you don't have a networking issue?

If it were a network problem then larger files would also transfer slow.  But I just tried a 30GB file, and it immediately started transferring around 90MB/s, then a 600MB file went at 40MB/s.  This is copying from the same drive on my desktop to the same directory on the server.  I don't understand the discrepancy in these numbers at all.