If you can, avoid bringing a bag to Epcot. That security line is the worst. The other three parks seem to go pretty quick as they have many bag check lines. Epcot's security area is small and they can't fit any more tables in there.
For us, the number of security tables/lines wasn't the issue ... it was how thoroughly they were checking everything! It took them forever. I know it's for our safety, and I appreciate that - but Epcot is the only park where they opened every single zipper on my backpack. I carry a small backpack, and don't keep things in every zippered area. I open the zippers that have things in them, at all the other parks they would just feel the pockets that weren't open, realize there was nothing in there, and move on. But not at Epcot! Every. single. zipper.
And, the major reason it went slowly, is that folks were not ready and taking ridiculous amounts of stuff in multiple bags with tons of pockets they didn't have open or multiple shopping bags. I was always amazed.
This. It always frustrates me how people can stand for 5 minutes, get up to the front, and not have their bags out of their strollers, or zippers open, or just not be ready at all!! If there are longer lines, I always stand back and assess for 30 seconds - and pick the line where people look like they're getting ready properly and will make it through faster. It usually works, and I'm through the line before others who were at the same spot in other lines as me!