I think speed of delivery probably has more to do as to what time of the day it is, what day it is, and whether the package ends up at the front of the Semi or back of the Semi and how far away the main hub of USPS, UPS, FedEX is. Just one note don't expect much time wise if you order something on a Saturday, because I think all the above shippers have Sunday off, and I know the facility I work at closes down for Saturday nights, during the non Christmas season.
Usually when I pick the slow free shipping they seem to sit on it for at least a day or two just to make sure I know what slow free shipping is. But there was that stretch last year where everything was next day.