Ok, need to see if you can isolate the router from all the others near by if you can. I would see if you can find another open channel if possible. That many routers near by, possible cause of connection speeds. I would test the router with one or two devices, preferably if you have a 2.4 and a 5Ghz device to see if you can get fast speeds on each radio. If you can't find a open channel. I might try to lower the out put of the radios from High to Medium and test to see what speeds your getting in a same room, say 4-6 feet line of sight to the router. Test speeds on each radio with the two devices. I might take the router to a different location if possible, say family or some place not in the same neighboring area with all those routers. You don't need a internet connection to test the wireless speeds with devices. Just would like to see if getting away from those other routers effects anything.
I'm might also go back to v1.00 and test as well, if you decide to try this, don't load any configurations, just factory reset, downgrade and factory reset once more then test the wifi with one or two devices.
Is this the only router you have?
Let us know how it goes.