I call kids in groups to get their things from their lockers. I send one group at a time. I'll send the next group when only 1 kid is left in the hallway. They all know the rule is to get dressed in the room, then sit in your seat. When they are all sitting and ready to go, then I let them line up.
I do send the slow ones out to their locker first, giving them 10 minutes to pack their things and get dressed. It seems to work for me. Good luck!
I have started to have everyone pack their things before they go out to the last recess. The pokey ones (!) are using their recess time rather than classtime. This works for me because my classroom is on the playground side of the building and our last recess is a 1:00. I do have to have all necessary papers in their cubbies ready-to-go.
I have had a number of Dilly-dallies & I finally got tired of telling them to hurry up, etc. Even if these few aren't ready yet, the others are fully dressed with gloves, mittens, etc & are getting HOT so I started lining up and leaving the room without them! (Of course, I led the class down the hall v-e-r-y slowly.) This motivated them to pick up the pace & get moving! I have fewer dilly-dallies now .