file copy speed extreme slowdown after that nic card has ICS enabled

2 windows 7 PC involved direct conneted to each other gigabyte nic cards. I booted up win7 on the ubuntu PC (dual boots) And did a large file copy from client win7 to other win7 that has internet. Results were close to 70 MB/sec Now it is terrible slow AFTER I enbled ICS on that nic card to share the internet to other win7 PC. So please explain to me how doing that would kill file copy...