Hi,
A delay generates a phase shift.
While the delay is constant, the amount (in degree) of phase shift depend on frequency.
Imagine a delay of 1us.
With a 100k Hz signal a full wave (360°) takes 10us. So 1us equals to 36° phase shift.
With a 10 kHz signal the same delay equals to 3.6° phase shift
With a 1MHz signal the delay equals to 360° phase shift.
Delay within a regulation loop is often problematic for stability.
To stabilise a regulation loop with considerable delay time often needs to attenuate feedback signal, but this makes a regulation loop unprecise and slow. Best is to keep delay times low - but at least constant. It should not vary with time, temperature, supply voltage...
Klaus