That's a misconception. Having mixed mode doesn't reduce your speed to the lowest standard. B devices will connect at b speeds, g devices will connect at g speeds, n at n speeds. N speeds will only work if WPA2 is configured as security.
Are you located in a crowded wireless neighborhood, meaning do you see a lot of other wireless networks when you search from your device?
If yes, try changing the wireless channel. Try different ones, reconnect your device and see if you get any improvement.