There is no delay in the IQ Panel from when armed before a motion detector can trigger (other than exit delay), but there is a delay in the motion detector itself.
This is a common question because the results can throw people off guard, here is a similar thread.
Battery operated motion detectors do not continuously report motion when it occurs. To save battery life, a motion detector will only detect motion again after a reset period of no activity, typically this is about 3 minutes.
Try staying out of the motion detector’s view for a few minutes, then test. It should activate right away when you walk into view.