@KasaUser Yes, I have a Deco X55 as my main unit, with 2 M5 sattelites (running 1.4.0). Honestly, I've been disappointed with the X55 since making it my main Deco unit. The issues of slow throughput and devices not following their connection preferences seems to have gotten more consistent since upgrading the X55 (HW v 1.26) to firmware 1.2.0.

I think I'm going to make one of the trusty M5s my main deco unit again. I had the X55 as an ethernet backhaul sattelite before and the network seemed to perform better and more reliably with the M5 as the main unit. TP-Link advises using the X55 as the main in my scenario, citing upgraded hardware, but from what little info I can find online, if anything it might perform on par with the M5. B&H photo (since TP-Link doesn't answer this question) lists the M5 as sporting a quad-core CPU at 717MHz, while the X55 only has a dual-core CPU, running at 1GHz. I suspect the two extra cores in the M5 make it a better performing unit.