Upgraded my tp-link C2600 from 18.06?.x? to 19.07.4, wifi AC slower now


I upgraded my C2600 from 18.x to 19.07.4 yesterday, and I just noticed that my 5ghz AC actual measured speedtested throughput is quite a bit worse than it had been previously. I kept my configuration the same. Can anyone suggest how I might go about troubleshooting this or offer some tips?
EDIT: watching my device in overview, I notice it cycling from this:
to faster speeds/width and then back. I think this is actually normal, but .. just in case.

The AP will always revert to the (lower-) base rates on an idle link, but should ramp up to full speed once there is demand (from your clients).

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Right thanks, I won't worry about the displayed rates then. But my actual throughput is also low.

You could try reverting back to stock ath10k firmware, see this post for an example:

Just make sure you do this over a wired connection. In your case the firmware package name should be: ath10k-firmware-qca99x0-ct

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OK so in desperation, I recreated the main SSID, and performance was good!
That led me to test the alternate SSID's which were also fine.
I went through the various settings until I found the culprit. Having WMM off on SSID main (and on for guest/etc SSID's) drastically reduced my throughput suddenly.
I don't suppose anyone has any insight on why that would be the case? Is it because of the mismatch?