Dave Plowman wrote:

Not having such a device, presumably they introduce a delay? So if you can
hear the same prog from another room, a PITA?

Depends where the source is, tends to be for online streaming rather
than a "relay" for an in-house device. With the chromecast you can set
up groups of devices, then stream to an individual device or a group.
There is the ability to add a delay to individual devices, but I've
never needed it, it might come into play if multiple wifi access points
or repeaters are in use I suppose.