I did a lil' searching on this forum and similar model NICs (5100/5300) have the same issue (with the same router). The fix lies in enabling WPA2/AES or enabling QoS or enabling WMM or a combination of two of these or all of them. I haven't had a chance to try any of these fixes yet though.
This information from another thread may help you.
Finally got this figured out. In case others have the same problem as I did (i.e., PC wouldn't connect at more than 54Mbps/wireless-G speeds) and want to avoid hours of frustration and deadends... To summarize what I did to get this to work
1. Have current Intel driver (on their website); Lenovo had not installed current driver on my new laptop; oh well
2. Set router to 20/40 Auto channel width, mixed n/g
3. Turn on WPA2/AES encryption
4. Enable WMM and QoS on router (QoS was already on for me, but people on Intel forum told me to check it was on so sure it must do something good)
Step 4 worked magic - went from 54Mbps to 270Mbps in a hurry. Speed fluctuates a bit it seems but still a vast improvement.
Ya, I did the same thing except I'm only getting 130 while my usb adapter still gets 270. ********!