Google Duo seems to be unusable on my 1ii and 5ii. I've tested on multiple devices, but only these two have issues. Calls have poor quality, but also 3-4 seconds of lag. Sometimes the video seems to speed up to catch up but it's never down to a decent level of delay. 

It's worse when on WiFi. It gets better after a restart and worse the longer the phone has been on. 

WhatsApp video calling seems to work fine. 

I've tried reinstalling the app, phones are up to date, home WiFi is good (350Mb with a mesh system, no other apps have issues).

Other Xperia devices on the same WiFi and using the same Duo account have no problems, it's something specific to the 1ii and 5ii. 

Anyone else struggling with this?