As a general rule all of the affordable mesh systems will have slower performance from the secondary nodes, for a number of reasons. A notable cause is that most less expensive systems (like the MW6 and my TP-Link Deco M4) are dual band, not tri-band that you'd generally find on more expensive mesh systems. Other considerations that apply whether the system is tri-band or dual band are how many nodes there are, whether they've got strong connection to the primary, distance to the primary etc. In some cases you may be able to improve matters by running an ethernet cable between the primary and secondary nodes - but trial that before going to the effort of actually fitting the cable all round the house - the cost of a long Cat 7 cable probably isn't material, but the effort running and permanently fitting it tidily usually is. However, the purpose of mesh systems is to offer seamless coverage with speeds adequate for all reasonable purposes, rather than attempting to get ultimate speeds. So, your options:
1) Accept that seamless coverage with MINIMUM speeds of 110 Mbps is excellent by most standards, and the system is working as expected.
2) Play with placement of the nodes, maybe trial an ethernet link between primary and secondary, perhaps experiment with QoS settings and see what you can ring out of the system. This could be a bit of techo-fiddling-fun for a rainy day, perhaps.
3) Sell the MW6 on eBay (mesh systems command decent prices, so you shouldn't lose much) and buy a premium tri-band mesh system. But you'd still probably see a far more modest fall in performance when connected to the secondary if its a pure wireless mesh. However, if you're that desperate for the performance, and are willing to spend the money I'd say be an early adopter of Wifi 6 mesh, which are just coming on to the market. These are about 50% more expensive than an otherwise comparable 802.11ac mesh system, but are probably a wiser buy now than investing in any 802.11ac mesh. Take a look at coverage of TP-Link's Deco X20 to see what the makers claim as an example - that seems to be in distribution for about £280 for a three node system. Obviously you won't get all the benefits of Wifi 6 until the client devices support it, but premium devices have supported this for a couple of years now, and it will cascade down as products are refreshed over time, and in the meanwhile the Wifi 6 mesh should still be better with "ac" devices.
Regarding bridge mode and clashes with the Sky Q, sorry, not got a clue.
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