Finally I got 300Mbps Wifi speed at home between my laptop and the router. This means a theoretical 30MB/s for file transfer with other computer or with the NAS. In reality I get 16MB/s instead of the 5 or 6MB/s I was getting before. It was not easy to understand all the components that made so difficult to get it working. Here are my collection of tips for configuring the router:
1. Use WPA2-AES encryption
This type of encryption seems to be needed to have speeds above 54Mbps. But I could not find information if you can also get higher speeds by removing any encryption. In some forums say the 802.11n specification obliges the use of WPA2-AES encryption, but here it seems to say that you can get a 5 to 10% gain by removing encryption.
2. Use 40MHz bandwidth
It means it bonds two channels of the spectrum to transmit data. I did not knew about this until I saw the option recently on the router of my new internet provider.
3. Set the router to use a channel with free adjacent channel
This was the most difficult to find out about and the reason why I was on 144Mbps for long time. As I live in city center there are more than 10 networks on the range. Also I had always read that it is good idea to choose either channel 1, 6 or 11, not to have overriding channels. It comes out that every network in range uses one of those three channels. When I moved my network from using channel 11 to use now channel 8 (plus channel 7 or 9 as adjacent channel, the configuration does not allow to choose), my connection changed from 144Mbps to 300Mbps!