AirPlay uses a fixed latency of 2 seconds, so 500ms is not bad. Either way it's not a technical accomplishment/failure, the longer the latency the longer you can put up with your WiFi breaking without having your audio blip.

500ms should be enough that you can roam between multiple APs without the audio stream being interrupted. If you're on hard-wired ethernet you should just be able to modify the snapcast source code to reduce it to whatever you want (10ms should be fine).