I have the Deco M5 with 4 total decos connected to my network, including the main deco unit. The main unit is connected directly to my ISPs box and we have a fiber connection. At the time I setup my network, my ISP contract was for 25 Mbps download speeds. We were having issues with gaming, so I set up QoS optimized for gaming and used a speed test to set my Total Bandwidth. The speed test showed ~28 Mbps down.

After upgrading my ISP contract to 75 Mbps download speeds, I noticed that my connected devices were still limited to 28 Mbps. Connecting directly to the main deco via an ethernet cable showed I was capable of getting 80 Mbps using a third-party speed test, but wifi connections were never exceeding 28 Mbps. I ran a speed test via the Deco app and saw that my main deco was also getting 80 Mbps to the ISP, so I knew it was something to do with Deco throttling connected devices. After a lot of fruitless Googling, reading this forum and the frankly terrible TP Link support articles, I decided as a last ditch attempt, to muck around with QoS again.

I opened the Deco app and the QoS settings and checked total bandwith. On that screen I saw the results of my most recent speed test selected, as seen here:

https://imgur.com/a/vA0ceSW

I decided to click "Test" again. This re-ran the speed test that I'd done earlier outside of the QoS screen with the same results, about 80 Mbps. I pressed save and got back to the main QoS screen. I decided to run a third-party speed test from one of my devices for the heck of it. Boom, 80 Mbps.

PSA: Deco M5 QoS throttles connected devices based on speed test saved when setting up QoS

YOU MUST RERUN YOUR DECO SPEED TEST FROM WITHIN THE QOS SCREEN TO UPDATE QOS BANDWITH SETTINGS.

Absolutely atrocious app design. Hope this helps some people from tearing their hair out.