It sounds like something is either scanning it on the fly as MarkV said or possibly something is just useing up all of your cpu so that your computer cant process it fast enough but i think that would be unlikely. also is the 100mbit network cable or wireless, wireless transfer speeds are alot less then cable but should still be faster then what you have said.
I would suggest tempoarily disabling any antivirus/firewalls to see if it transfers faster.
