Timing lyrics so people can follow
Lyrics have to appear before they are sung, not as they are sung — a line arriving on the beat is already too late for anyone trying to read it. Around a second of lead-in is the convention, and it is what makes commercial karaoke feel effortless. Show a line at a time, or two at most, and hold each one until the phrase is finished so the singer is never left mid-sentence with nothing on screen. The preview is where this gets settled; timing that looks fine as numbers rarely survives contact with the music.