Ok, I will try to be more clear
Router : D-Link DIR-625
Wireless adpater : Linksys AE2500 (USB card, dual band 2.4/5 GHz)
When I'm not using the wireless, in the windows task manager, in the Network tab, the link speed show is over 100Mbps. When I start using internet, like a youtube video or a game patch, the link speed drop to 5 or 6 Mbps and the usage raise to 100%.
I would like to go to a 5GHz router but it is out of my control right now :S
Thanks for the reply