If the files are encrypted on disk, there isn't any difference in workload and there shouldn't be much difference in transfer speed. If the transfer itself is encrypted, both of your models have hardware accelerated encryption which can easily saturate a Gbit link. In fact, they're rated to up to 225 MB/s with on-the-fly encryption.

Transfer speed drops dramatically with smaller file sizes. This sounds in line if you're moving a bunch of smaller files (a few megabytes or smaller) then your speed will suffer. There's really not much to be done about it, it's the nature of file systems. Each file has to be created and that interrupts the stream because most file systems don't multitask.