We in the mortgage indusrty all know that anything near 15 days just about never happens these days. Though some make make those claims still, it does not mean it ever happens. Or at least on any low interest rate and low closing costs loan.
The Dodd/Frank Act has made it extremely tough for any lender to move quickly. (excluding Hard Money, who can close under 15 days)
You'd be very hard pressed to find anyone who has closed in under 20 days from day of application, to date they closed...and received a low rate.
Call around to lenders as a prior Loan Officer asking for a job and ask them after a few initial questions, BTW if they can close in 15 days and they will all laugh!
If you want the best terms you need to allow for closer to 30 days