Screen casting/mirroring between Nexus devices and Samsung TV

I have a Nexus 4 and a Nexus 10, both running latest version of Lollipop. I just bought a H6400 series Samsung TV, which supports "Screen Mirroring" aka "AllCast" in Samsung terminology.

If I try to cast the screen from the Nexus 4, it just says connecting for ever and nothing happens.

Trying on the Nexus 10, it does connect but is quite laggy and often drops the connection. When it manages to keep the connection, the internet wifi speed on the 10 goes from 20MB/s to 0.2MB/s (using speedtest.new), which makes the internet unusable. This is with all devices (inc router) within about 3m radius.

I have an old 802.11bg WiFi router, so was going to upgrade that, but have found out that screen casting uses a direct WiFi connection between the device and TV, it doesn't use the WiFi router at all.

Does anyone know how to make it work with the 4? And with the 10, is the WiFi becoming unuseably slow a common problem or anyone have a fix?! Would upgrading my router to one that supports both 2.4Ghz and 5Ghz help? i.e. can I run the screen casting over one freq, and the internet wifi over another one?