I see there’s a large number of posts about the Duo being very slow over a local network.
While this might not account for all instances, I wanted to contribute that the router that ships with Plusnet (UK) broadband - in particular with fiber (the Technicolor 582n) [Deleted] and has had nothing but complaints.
People who have had this router have reported VERY slow and unstable WAN and LAN speeds - unusable in fact - and once replaced with an alternative, the speed issues have been resolved.
Myself included. I changed to a TP Link TL-WDR4300 and ALL of my network problems went away.
Hope the info can help someone else.