I recommend using IE for testing speeds. Were seeing many different issues using Chrome and is having adverse effects on these routers.
Always test your internet speeds using a wired connection. Using WiFi can not achieve the same results due to many variables on Wifi and external conditions and device configurations.
Your connections are only limited by what the devices are supporting, so some devices can use 5Ghz some can't. 2.4Ghz will not achieve the same speeds as 5Ghz. The connection speed is higher. Most speeds you should see should be fast however not as fast as 5Ghz or even Wired. Wireless regardless of band, will not ever achieve the same xfer speeds as wired connections. One the other hand your devices should be connected at the maximum connection speeds with the router. This is different from xfer speeds. You should be seeing anything from 54, 65, 130 to even 300 if your devices support that speed. Another thing to consider is using AES only as a wifi security option. It has the best performance of the 3 however some wireless devices don't support AES. Also using bandwidth 20Mhz can impact connections however Auto 20/40Mhz should work well for most devices.
2.4Ghz has better range than 5Ghz as 5Ghz works best in line of sight or maybe one wall room away. Any thing between the router and clients on 5Ghz will effect 5Ghz signals. Thats just the nature of the 5Ghz beast.
Any other WiFi routers in the area? Link> Use
InSSIDer to find out. How many?