In the U.S., a payment from a bank account will go straight through only if a credit card is listed as a backup source. If the account doesn't have a credit card, it takes four or five days for the payment to clear.
Or to be even more technical, it takes four or five days for the funds to be available inside PayPal for you to draw upon them.
The transfer happens almost at the speed of light. The money is no longer available in your bank account. PayPal (and everybody like them) will then wait a few days to make sure everything is legit.
This used to make sense when checks and transfers traveled around the country as pieces of paper. There's legislation brought up from time to time in the U.S. and elsewhere to eliminate the long "hold" on a transaction that was completed electronically. Because, during those 2-4 days, you aren't using that money; your bank isn't using that money; only PayPal is. A significant part of their bottom line comes from investing the "float" of money that is in their hands but not yet disbursed.
I was delighted with the explanation by PayPal's predecessor, X.Com (which was later bought out by eBay and renamed PayPal to replace their home-grown version). One of their FAQs was "How can you afford to provide those basic services for free?" and the answer was "We make money on the float." If they use $1000 for four days, they can earn about a buck. It adds up.