[CentOS] Jumbo Frame performance or lackof?

After successfully getting higher MTU to work on my Realtek NICs, I
started testing the impact of higher MTU on file transfers using NFS
exported ramdisk to ramdisk.

The results were unexpected. The higher the MTU on the sending NIC,
the lower the file transfer speed. I tested by using time cp to copy a
1GB file (In case compression might affect the results, so I dd the
test file from the CentOS DVD ISO)

The average of 6 runs per MTU with varying MTU on the receiving NICs
MTU 1500 = 116.34 MB/s
MTU 3000 = 103.42 MB/s
MTU 4000 = 102.52 MB/s

I was actually planning to do 1500, 3000, 4500, 6000 and 7200.
However, I was surprised to discover although I had two seemingly
identical Realtek PCI-E nics, the sending NIC could only go up to 4K.


I'm wondering, that since Jumbo Frames was supposed to be better for
bulk transfers, why am I seeing these results? Is it the ElRepo
drivers I used to enable higher MTUs or possibly some kind of oddity
with the realtek NICs I am using? Or am I mistaken about the benefits
of jumbo frames and that they are only beneficial in specific
configurations, perhaps only with higher network speeds and enough
packets at 1500 MTU to overwhelm packet handling hardware?