Got me beat!
I've now experimented by bouncing back and forth between the 2.4 GHz and and 5 GHz bands several times, and since my first venture back to 2.4 I've gotten similar throughput from both.
I doubt that my issue had anything to do with outside devices, spurious or otherwise...it was consistent for too long, and then it just disappeared.
Being that the problem began when I upgraded from Snowy to El Cap and disappeared when I switched bands suggests to me that something in the Base Station needed "flushing", but since I know absolutely nothing about its internal workings, that's nothing more than a
very wild guess.
As I have said before, with the complexity of all of these things, the fact that they work at all, or as often as they do, is amazing to me.
I've said the same thing many times, Ira; sometimes I sit here scratching my head...in awe of it all.