There is a body of literature on emergency aircraft evacuations, and a quick google search found this - http://www.faa.gov/data_research/research/med_humanfacs/oamtechreports/2000s/media/0402.pdf .Studying evacuations in a controlled setting, they concluded that three variables that affect exit time of passengers are (in order) waist size, gender, and age.

While flying is a very safe mode of transport, accidents do happen and one should prepare. I typically carry the most significant stuff on my body, in my pockets or within my flight vest (it has not been used for any purpose in years) If the aisle ahead of me is clogged with large, old passengers squeezing toward the exit (highly likely), I might take the time to scoop up my very small, well equipped backpack.