Hi Tom: The 'timing intervals', number of parts per quarter note (ppq) is frequently refered to as "timing resolution". What this essentially does is quantizes the timing to a specified portion of a beat: 64 ppq = 64 parts per quarter note, 480 ppq = 480 parts per quarter note. The higher the number, the finer (higher) the resolution and more musically accurate it sounds. I somehow doubt that the SD1 'fill in' delay is caused by low timing resolution, but it may be something to consider though.

Luka: please tell us exactly WHEN (what measure, beat and fraction of that beat) you are pressing the fill-in button and also, exactly WHEN (what measure, beat and fraction of that beat) the fill actually begins. - Scott