Marper - sorry I don't have a working system at the moment to work this through, and as I say, networking on Linux is not my comfort zone!
However, I can say that you definitely appear to have a significant network problem from these stats... you are getting less than 5Mbit/s with one large file, so lots of small files don't stand a chance. Given this was a simple file copy, the LMCE doesn't seem to be implicated in any way. It could still be the speed of your systems, but very unlikely as the data rate is SO low, and as totallymaxed pointed out, these processes (booting) are not particularly CPU bound.
Another point is that your file transfer is TCP, but the boot process is UDP, so it is unlikely to be TCP congestion control issues, which eliminates the network stack, leaving us with the NIC drivers and the network itself.
The next "high value" test you should do is bypass the switching and network cables with the cross-over cable direct from NIC-to-NIC (core to MD). They are only a few bucks from your computer store. Do that and try the boot again.... we can go from there...