Streaming to a Speaker Group

[majones]

I'm running the server on a RPi4 and it streams straight away to single speakers, but when I stream to a group (2 Echo's and a Sub) there is sometimes a wait before the music starts (initially and between tracks), and sometimes it doesn't start at all. I've experimented a bit, and if I load the server onto a fast Windows machine, the stream to a group more often than not won't start at all. Is there something about group steaming that I need to take account of on the server or on the LAN?