thanks for the quick reply, and apologies for not leaving so much info in my first post
Here are some additional infos:
Hw version: B5
Firmware: 2.10
I have cable. I only have one router, the dir-600. No additional one from the ISP, only the network cable that's plugged into it.
I obtain the ip automatically so is DHCP.
all Qos are disabled
DNS relay is on
tried single mode ( n only) and mixed - same result
security wpa/wpa2 with cypher type tkip/aes
...still 65 mb/s
strange is that before I have upgraded my wireless drivers, I had 75mb/s. I use a laptop ( asus g73jw) with atheros 9285 wirless card. last drivers are from November 24th.