I had a four unit deco M5 network that was working ok but had some dead spots.

I installed a new Ethernet cable which gave me the opportunity to re-jig everything and buy 2 deco x90's.

the cable speed tests at around 800-900 mbs down/up so working well.

I have one new x90 set as the primary and the other x90 and m5's as slaves.

the problem is that as soon as one M5 is added to the network, wi fi speeds for everybody drop to 50mbs.

the only exception is if you connect to the primary x90 then it's working as fast as expected.
 

if I power off all m5 units speed for both x90's is ok.

what am I doing wrong as I can't believe an m5 connected to m5 primary works better than a x90?!? 50mbs vs 250-300mbs