KK959:
You may think that I can just use the delay to fixed this problem. But my project is a arpeggiator with real time control and real time display, if the delay is too much, the playback will be sluggish !
Don't use the delay() function. The demo Several Things at a Time illustrates the use of millis() to manage timing without blocking. It may help with understanding the technique.
Have a look at Using millis() for timing. A beginners guide if you need more explanation.
...R