My DLink DIR 655 wireless-n router (set to use wireless-n mode only) used to connect to my laptop (Intel Centrino N-1030 wireless card) at 150Mbps. I added two wireless Access Points (both wireless-g only) to the router to extend range. Now my laptop connects to the router only at 65Mbps even when there are no clients connected to any AP's. I thought the wireless throughput reduction in mixed n-g network only occurred when wireless-g devices were operating...??? Does simply connecting a wireless-g AP reduce wireless-n throughput??
One of the other access points is using the channel space the DIR 655 was previously using to reach 150Mbps and it has now had to reduce its bandwidth to avoid stepping on the other access points. You can change things so the DIR-655 has two consecutive channel blocks all to itself and the speed should go back up.
With a single spacial stream, 802.11n tops out at 65-72 Mbps in a 20MHz channel and 135-150 Mbps in a 40MHz channel. So the numbers are perfect. (If you follow the link, look at row 7.)
Unfortunately, there just isn't enough space for two 802.11g systems and one 802.11n system using a 40MHz channel. The 2.4GHz block is only 60MHz wide. You'd have to put the two 802.11g access points on the same channel, either 1 or 11, to leave a 40MHz block in the middle.