I recently discovered a huge performance decrease in download speeds and eventually narrowed it down to the way my wireless routers have been configured.
As a note: my wireless routers are not used as routers but as access points and bridges.
Basically, you have to leave WMM support enabled in order for most 802.11n wireless cards to connect to an access point supporting 802.11n. But in my scenario, a wireless router configured as a bridge was the client. WMM is an extension of 802.11e, which is Quality of Service (QoS) for wireless protocols. The N-wireless standard requires that this is supported.
The important thing I discovered was that the bridge client needs to have WMM support disabled or download speeds will be less than 1 Mbps (as measured by http://speedtest.net) while upload speeds were normal for my ISP (Comcast Cable). I’m not exactly sure why, but it makes sense that the wireless client should not be setting WMM priorities over top of the access point’s priorities. It just confuses the communications since “upload” from the AP means “download” for the client and vice versa.
So yes, if you have wireless bridges & APs, especially those running custom firmware, be aware of these settings.