do you mean load time or play time?

I have a 256 card with almost 100 audio CD tracks on it. The load time from putting it into the SD slot until ready to play is about 5 seconds. The play time is instantaneous and depends on the silence you record at the beginning of the tracks, i.e. if the tracks starts after 2 seconds of silence, it will start 2 seconds after you press the play button. If you edit the tracks for zero silence at the beginning then playback is instantaneous when you press the play button. Likewise if you want to skip 10 tracks forwards or backwards while playing, the new track starts to play immediately after the tenth press.

Real background vocals is an ideal application, I have heard a vocalist duet live with herself from SD in a large concert hall and the clarity was excellent at high volumes over a large high quality pa system.