For most banks this is not the case. Transfers within the bank are usually instantaneous.
It is not uncommon for banks to draw out the length of transactions because while the money is "transferring" or "settling" it is actually sitting on the bank's balance sheet, being lent out but not earning any interest. A good deal for them when you aggregate over the millions of customers they have. Your bank may be trying to squeeze a few pennies of interest out of you.
Delays in transactions also allow their fraud team the flexibility to investigate transactions if they want to. Normally they probably don't but if the bank delays all transactions, then those being investigated will not be aware of it.