Slow 2.4GHz on WRT1900AC with OpenWrt 18.06.2

Current latest OpenWrt 18.06.2 mwlwifi on WRT1900AC v1 mamba has problem with slow 2.4GHz throughput which is not stable (getting max 20Mbps on auto ch11 and for some manual channels max up to 33Mbps unstable. While on ch5 throughput is terrible 0.3Mbps. Although setting channel width 20vs40MHz does not affect throughput the problem, but when I enable in Advanced " Force 40MHz mode", I get ~58Mbps on ch3!? which is why I am running it despite saying that this does not comply with IEEE 802.11n-2009! which tells me this it just a hack and not a solution.)
This is NOT an issue with channel congestion, since I have tried and used few other routers that fully saturate 2.4GHz N at all channels.
So far I did not notice any problems with 5GHz although did not run any extensive tests there yet but so far all of more than dozen devices show good 5GHz performance.
Versions of mwlwifi are:
kmod-mwlwifi - 4.14.95+2018-11-14-81413aa9-1 - Marvell 88W8864/88W8897/88W8964/88W8997 wireless driver
mwlwifi-firmware-88w8864 - 2018-11-14-81413aa9-1 - Marvell 88W8864 firmware

Question/ask: What mwlwifi is known to be stable without such problems? (or when this will be released or how could this be set up to work well or is davidc502 or eduperez which has older build better,..?)

Not the case.

Some routers allowed for a "forcing" of 40 MHz (this basically means its beacon announced it was capable, not that it would jam the band by actually doing so when congested). This standard changed in 2009. If your device has the option, that means its WiFi card/chip model was capable of performing that setting prior to 2009. That's all.

Also be advised, just because the AP does so, doesn't mean a client will actually switch into 40 MHz mode during congestion on the 2.4 GHz band.

Regarding the driver specifically, I'm not familiar, I am aware some settings do not work - someone more experienced with that driver - will hopefully chime in.

Hope this helps.

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Possibly issue308, there is PR1887 open with that change, not sure of @eduperez has a 18.06.2 mwlwifi build with that change incorporated which you could try.

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I'm currently (trying to) build packages with that commit, but for some reason the compilation fails for me... just give me a couple of days.

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It turns out that after few days of using force 40mhz, it actually rarely works, which aligns with what you pointed.

I just tried your last build:
opkg list_installed|grep mwlwifi
kmod-mwlwifi - 4.14.95+
mwlwifi-firmware-88w8864 -

Result is that it behaves the same as original boundled with 18.06.2 giving slow unstable 2.4GHz ~20Mbps