Well that's an interesting find there, I have never personally noticed a huge difference. If I had to guess safe mode is loading generic driver
types while full boot is using the assigned proper driver you have loaded for a device. At first glance that's the only thing I can think of that is affecting the net speed. There is no service that should be consuming so much of your network resources that it represents a 75%+ decrease in network performance.
All I can really offer is that explanation though. Hopefully someone else will have something to offer you in the way of a actual fix.