After examining the wpa_supplicant log:
a0:55:4f:66:d5:18 freq=2447 qual=0 noise=-89~ level=-49 snr=40* flags=0xb age=0 est=65000
a0:55:4f:66:d5:10 freq=5240 qual=0 noise=-92~ level=-67 snr=25 flags=0xb age=0 est=390001
The wpa_supplicant people say:
According to the wpa_supplicant AP scoring, it would indeed seem that the 2.4GHz AP is prefered since it has better SNR (although the SNR for both is good. Generally, wpa_s considers any SNR above 30 as good).
perhaps the scoring can be tweaked to more aggressively favor 5.2 in case of close good SNR.
At the place where I usually sit/work, about 10m from the wap with a structural wall in between, wpa_supplicant almost always chooses 2.4ghz.
Using iperf to test, I got 4x the throughput when I forced wpa_supplicant to choose 5ghz.
Anyway - it would seem the issue is not with wifi.sh, but with wpa_supplicant.
This been said, wifi.sh does give the impression that it is going to connect to 5ghz, when in fact it allows wpa_supplicant to choose between 2.4ghz and 5ghz.