There is also no reason to have a banner 5' x 10' to have a resolution 300dpi. You do not view a banner that large up close where eyes can resolve down to 300DPI. 100DPI would be good and cut the number of pixels Photoshop has to process
At 100DPI the image would be 6000px by 12000 at 300 DPI the image would be 18000px by 36000px that 72MP vs 648MP