Does synchronized playback suck or is it just me?

[snubber]

Not really ... but I suspect it is not a common situation.
Note, though, LMS is single threaded so perhaps it might occasionally be starved by something manager the VMs.

I didn't know that, but honestly that does explain a lot. Having exclusive real-time control of the hardware is probably critical with larger numbers of devices being kept in sync. VMs can have CPU I/O delays that wouldn't be experienced on bare metal.

[dmozell]

Maybe icecast shouldn't run on the same machine as LMS. I have icecast on one raspberry picking up sound from my turntable and CD player. Another raspberry with picore lms picks up the icecast stream and transports it to several raspberry squeezebox players in sync. Occassionally I have to restart the players but 95 % of the time it works great.

[sle118]

Maybe icecast shouldn't run on the same machine as LMS. I have icecast on one raspberry picking up sound from my turntable and CD player. Another raspberry with picore lms picks up the icecast stream and transports it to several raspberry squeezebox players in sync. Occassionally I have to restart the players but 95 % of the time it works great.

Just my two cents here, but I noticed you have a wireless network config similar to mine, running Unifi antennas. One of the biggest thing that helped in my case was to disable band steering which was systematically causing sync dropouts when the access point was trying to push either player into the 5ghz band. Unless you've already figured out the root cause, you should probably give that a shot.