I realize this is an old post, but hopefully I can help someone doing a search for "low speed". I have a DIR-625 rev A router.
To get the maximum throughput on a /n router and a /n laptop (or desktop), you need to use the WPA2 encryption scheme and the AES cypher.
For a long time I was sticking with WPA to keep accessibility with several old PDA's that were Wifi-enabled. The maximum connect speed I saw was 54mps with my /n laptop (even though I had the router set to Mixed /n,/g, and /b mode).
Upgraded the OS on the PDA's to be compatible with WPA2, and Zoom! My /n laptop now connects at 150mps+ every time! I can stream Hulu feeds wirelessly to my flat screen TV in the bedroom using the HDMI out port on my laptop!
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WyreNut