It's impossible to play festival without lag (even solo). Lag at the beginning, lag at the end; microsecond lag, 5-second lag; lag while dancing, and lag while being rapped over. It essentially makes it impossible to get a perfect score in almost any game (again, even solo). It's no use watching Messi in 4K perfectly vocalizing a Flo Rida rap if I average four 2-second lags per song (this got noticeably worse with the latest update, giving me lags of up to 5 seconds, where you hear what you have to play, but you don't see it lol)
PS: I'm using a stable 80Mbps Ethernet internet connection. The lag can't be due to Wi-Fi interference.