My setup looks like this:

BT Broadband Hub - wifi disabled, hard wired into a switch

Deco X75 mesh (3 units) - hard wired to same switch as router

Sky Q 2Tb - hard wired to switch (note, both wifi channels turned off via engineering menus)

3 x Sky Q mini boxes, all hardwired to switch (as main box, all wifi turned off on Sky Q minis)

It all works, however over the past 2 months channel changing takes between 10-30 seconds on mini boxes. Had an engineer around, we replaced main box and minis, still happened. Unplugged all Decos, the problem went away. Even with no mini boxes (or main) wired through a deco, just having the decos on the network causes this problem.

I've had this setup for over a year (initially put in to get around my Playstation Portal issues) and it's worked fine for the majority of the time. Any ideas? Sky's mesh is not great so that's not an answer. I'd prefer to not have to invest in yet another mesh system though. 

Thanks in advance!