That is very strange - I am getting the 112MiB/sec transfer rates (large files though).
Only last thing I can think of is that you have a network card, for which the kernel drivers are not integrated in the Rockstor installation, forcing it into some random network driver which limits the speed that way. And that could be fixed by installing the missing ones …
similar to something like this (there were a couple of other threads out there building on this one, too):