That did not take long.
I found a recommendation to set QoS/WMM on router to "Enabled".
For the E4200 it is under Application and Gaming / QoS / WMM Support = Enabled.
2.4Ghz speed jumped to 270Mbps
5.2Ghz speed jumped to 300Mbps
So I guess the related links that show up to the right of my post after I made it were of excellent help, although the one I clicked was not my adapter or router, the suggestion still applied.
I have seen a LOT of issues with the Advanced-N 6230 with no solutions... I suspect this solution of enabling WMM in QoS will solve many of these issues.
I have similar problem with Intel Wireless-AC 7260. At driver's Advanced Property list, I set Preferred Band as 5.2GHz. But it does not use 5.2Ghz band. Only very few MAC packets were sent out of 5.2GHZ channel. But it was very quickly switched back to 2.4GHz band. Its driver inf file only provides parameters of RecommededChannel24 but not channel setting in 52G band. Is anyone know how to modify its setting parameters for 5.2GHz band.
Thanks in advance.