It's been my experience and it has been explained to me that it is policy for any withdrawal to sit there in process for 3 business days before processing (in case you go bust and decide to reverse). Once that occurs, processing takes place on the following day.
The actual transfer of money occurs on the 5th business day and then it varies from 1-5 more business days to appear in your bank account. The first withdrawal I made sat in limbo at my bank for 5 whole days before crediting. The subsequent withdrawals only took 1 day at my bank.
I only withdraw on Mondays. When that happens, I have the money in my account by the following Monday. Like you, I at first thought this was painstakingly slow but then I remember my Full Tilt money