@Loximity I'm perhaps experiencing something similar. Speed on 5ghz is good. On 2.4ghz it's significantly slower with websites sometimes

not loading at all. As soon as I get within 5ghz range it's very quick again. I know there is a speed difference between the two frequencies, but it seems too slow on 2.4ghz, from my experience. What else is odd is that my phone will show strong reception on what I presume is 2.4ghz, but internet is just slow or not working at all.
 

The solution for myself is to add

more units and have 5ghz attainable all over the home.

A the moment I have an M4 3 pack. One of which doesn't work, but that's another story.

I've tested again. Downloading a file from my NAS on 5ghz and I get about 20MB/s. Switch to 2.4ghz and I struggle to get 1MB/s and it drops out to a few KB repeatedly.

@Loximity +1. Same thing here. Ridiculous slow speed at 2.4 and it is not the device. I have another router that works fine.

Same issue on my side. 5GHz works well, passing through the ISP bandwidth that is available (+-150 mbps) with low latency (20 ms). Using 2.4 GHz, I get 30 mbps at most and approximately 300 ms latency when standing just next to any of my deco m4 units. Btw, I am using 3 deco m4 units connected to an unmanaged gigabit tp-link switch (ethernet backhaul), with modem also being attached there (compal cable modem). The mesh network is set to the access point mode. The network operates over a detached house, se that occupied wifi channels are not a problem.

Any idea how to improve the 2.4 GHz wifi speed and latency?

@Loximity 

>In addition, it looks like the Deco prefers devices to connect to 2.4ghz, is there a way to resolve the link speed and band preference?

See this discussion: https://community.tp-link.com/en/home/forum/topic/210566?replyId=509026

Workaround suggested is applicable to M4 as well.