@mattyB 

So the current network diagram is main router--Main Deco--switch--other slave Decos.

What is the operation mode of the router and the main Deco? Are they all router mode? 

With main Deco connected to the router only, what is the speed you can get from it with your mobile phone or laptop? 

Please click here to verify the wireless negotiation speed in the meantime. 

Try to disable fast roaming and setup QoS as standard and clear all the high priority rules.

May it work and have a nice day.