Say you plunk down 600k at the auction in cash to buy the home. Your plan is to then refinance the home as soon as possible. Maybe pull out 500k in cash and have a regular mtg. Well you cannot do that immediately. You need to own the home awhile. For a cash out refi the standard is 6 months but that will vary with lenders. Also the value of your home when you do the refi will determine the limits of how much you pull out. The rate of the mtg will be whatever prevailing rates are 6 months down the line when you do the refi. Bond market dictates where mtg rates are.
