I have Deco M9 mesh, with 5 units. As I have sky FTTP, I am using in access point mode. The Decos seem to be limiting the speed. (I have 500Mb fibre)

For example if i connect a laptop to the sky Wi-Fi, I get about 400Mb when beside the sky router.

The main deco is connected to the sky router via cat 6 Ethernet. When I connect to the deco network, I get 50-60Mbps when beside the unit. 
 

if I connect the laptop straight to the sky router via cat 6 I get the full 500. If i connect the laptop to the main deco via cat 6, I am still only getting 50-60. So the deco is limiting the bandwidth?

If I take the cable that is connecting main deco to the router, and plug it into laptop, I'm getting 500Mbps. So the deco is definitely receiving full bandwidth from router. But via Wi-Fi, or even hardwired to deco instead of sky router, I only get 50-60mpbs. 
 

the speed test through the deco app also reflects dame 50-60

any suggestions?