I posted on the issue in 2016, a few guys had same issue and then I had a thread on BT forums which got around 20 pages of responses before the thread disappeared. Ridiculous really.
The 5A caps around 25 to 30Mbps on 5ghz, the 5B is around 10Mbps on 5ghz. Both devices perform incredibly well on 2.4Ghz, I often could get 50+ on 2.4Ghz and only 27ish on 5Ghz, odd really as 5Ghz should perform much better. Reboots always solved it, even on fixed wireless channels. I tried everything, I mean everything, then switched to a talktalk router I had, followed by the hub 6. Used the Hub 6 around a year without a problem, now on Sky.
Best of luck, as I say HH6 was a great hub in my experience.
Edited by ukhardy07 (Thu 23-Nov-17 22:23:16)