Unitary board

I don't understand that point either. In fact, in our SBL notes I suggest that two tier boards will be slower.

Yes even i was thinking that two tier board should be slower as it involves strategy direction from supervisory board and then it needs to be implemented by management board so it would be a slower decision making as per my understanding ...please correct me if i am wrong

You are correct - it will often produce two decision-making occasions which will almost certainly take longer.

I have read somewhere saying because the unitary board has fixed date for board meeting, so its slower. Is this the case?

Why does it have fixed dates for board meetings? Directors can hold meetings whenever they want to.