R8000 2.4Ghz connection really slow, 5GHz super fast

How fast is your Internet service?

If you saw your channel changing frequently while in Auto channel mode, then this is a sign that all of the channels in your area are congested.  This will affect your speed tests and is yet another reason to use 5 GHz that I forgot to mention: fewer people have 5 GHz networks.  You can try using Wi-Fi scanners, like vistumbler, Acrylic or even Netgear's Genie to find the best channel.

Another variable that can mess with your results is the speed test itself.  If you are using a website to run speed tests, then the results can be skewed by congestion in the Internet.  Ideally, you want to eliminate as many variables as possible.  Performing a local file transfer or, equivalently, using a test tool that runs locally between two home computers will eliminate the Internet as a factor and allow you to better gauge just Wi-Fi performance.

There are features on the router that can impact the overall performance of the router.  These include Dynamic QoS, Traffic Meter, port forwarding, site blocking and MAC address cloning, to name a few.  Turn them off if you don't need them.

DD-WRT is the Swiss Army knife of 3rd party firmware.  It's extremely flexible but one area is doesn't excel in is performance.  DD-WRT doesn't use hardware acceleration, so speeds will be limited by what the router's CPU can handle versus what the Broadcom chipset can handle.

TheEther,

Many thanks for the detailed answer.

The Internet Speeds (according to COX) is 300Mbps down and 30Mbps up - when using speedtest I get 388Mbps down and 38Mbps up (either wired or on WiFi 5GHz network).

Good point on the speed measure, I will try the local transfer between two computers on WiFi and measure that.

None of these features are enabled: Dynamic QoS, Traffic Meter, port forwarding, site blocking and MAC address cloning.

If DD-WRT does not excel in performance, then its crossed out of my options -

Currently 90% of my devices are in the 5GHz WiFi network, I only have the sound system (about 5 Bose SoundTouch systems), my WiFi printer, WeMo light switchs - all devices that does not require high WiFi performance is not an issue.

What got me really interested in the subject is that my PS Vita only supports 2.4GHz WiFi (yeah I know, **bleep**ty) and I was getting terrible WiFi speeds with it. Then I connected my Macbook pro in the 2.4GHz and noticed that my WiFi speeds were around 0.5Mbps down and 0.3Mbps up - even with the R8000 reporting that the optiimal speeds should be 600Mbps. I know that 600Mbps on 2.4Ghz in a myth, but 0.5Mbps menas something is wrong - that is why I started this thread here. 

Yeah, 0.5 Mbps is pretty bad.  Do you have anything that could be causing interference in the 2.4 GHz spectrum?  Baby monitors, old cordless telephones, Bluetooth devices, satellite receivers or microwave ovens?  Here's a good article by Apple.