wndr3300 looses mbps by half

The WNDR3300 does not allow for simultaneous operation of the 802.11n standard in both bands, which is typical of routers marketed in that era. You have to make a choice between 802.11n connect speed in the 2.4GHz band and 802.11a connect speed in the 5GHz band, or 802.11g connect speed in the 2.4GHz band and 802.11n connect speed in the 5GHz band. Does it negate the reason for dual bands - in my opinion - NO. The 2.4 GHz band has greater range (compared to 5 GHz) and as a result, when in areas where there is a high density of WiFi networks is more prone to interference from adjacent networks, the 5GHz band, because of the increased attenuation that occurs at the higher frequencies has less range (compared to 2.4 GHz) and as a result is less likely to suffer from interference. Although current routers do allow simultaneous operation in both bands, it's not something I would advocate - using both bands when only one is required simply adds to the "WiFi congestion" in the area - if you don't have an interference issue use the 2.4 GHz band for the range, if you have an interference issue, try the 5GHz band - if it can provide the coverage you need, use it.