Interbank electronic transfers don't take time. They are instant. Back in the day when you had to write a check, mail it, wait for the post office to deliver it, wait for someone at the bank to open your payment and then go through the process of "cashing" your check .... it took time. With electronic payments it just takes a few moments.
There is no legitimate reason for a bank to hold money and not post the credit to your account. But I can think of reasons why it's more profitable for the bank to drag their feet.