Oops!! My mistake.
Rebooted EA6350. Low speeds persisted....
I then decided to check speeds with two other computers and they were fine....
It was the laptop I was using that is mysteriously returning low speed test results all of a sudden. Never had issues before with this laptop.
I will return to using 'performance' setting on EA6350.
Update: It looks like the issue I am having with some old Intel wifi cards (6205 and 6300). It is NOT related to the bug in hostapd v2.9.
https://forum.openwrt.org/t/zyxel-nbg6617-ipq4019-regression-in-wifi-5ghz/50544
I flashed an old 19.07-r10293-snapshot to my EA6350v3 and speed tests no longer capped at 12-15 Mbps with Intel 6300 wifi card on 2.4 and 5 GHz. All OK.
Update 2: Discovered a buffering problem old Roberts Stream 83i internet radio with 802.11 b/g wifi. OpenWrt reported downstream link speed was only 2.8 Mbps (upstream was usual 54 Mbps). I've now abandoned using OpenWrt as a pure AP, and returned to using 'Access Point' mode in LInksys OEM firmware.