The Delay slider determines the how quickly the LFO’s depth will reach its maximum strength after a note has been triggered. This allows the modulation to be introduced gradually, for example, to achieve delayed vibrato.
When the slider is set to maximum, it will take approximately 5 seconds for the LFO depth to reach maximum value.
The LFO Delay always works in a “legato” style trigger mode, where subsequent legato notes do not retrigger the start of the LFO delay. The LFO Trigger Mode has no affect on the Delay slider, since it is always in “legato” triggering mode.