AE here.. I wonder if you ever got a usable translation after all the moralizing and questioning your reason for asking...
I'd forget snail because although snails move slowly, they rather glide very slowly, don't they? Also, snails are a bit "cute" somehow, aren't they? Which is not what one feels like calling anonymous people who are obliviously blocking others who are in a hurry. So, it misses the effect of doddering old geezers, for instance. Oldsters, unstable on their feet totter or dodder. These words specifically mean to go slowly and unstably on one's feet, be it due to physical or mental infirmity.
To dawdle means to walk slowly, perhaps even stopping and starting, without hurry and probably also without much regard for others around you. A person who dawdles is a dawdler. Dawdle does not imply any specific age. Kids and oldsters and anyone in between can dawdle.
Note: Not to be confused with a toddler, which is always a child just learning to walk. "Mikie is just now learning to toddle."
These terms are not derogatory in and of themselves.
@dude mentioned slow poke, which is correct, but perhaps slightly quaint or lacking much in the sense of impatience or annoyance.. It's certainly not derogatory. Just a bit mild-sounding if one were actually stuck behind these people on a sidewalk or in a car and feeling impatient to move ahead. A young, quick moving person might say to say to his/her friend:
"I wish all these doddering old farts, oldsters (or dawdlers) would just move out of the way and let us get through."
One's mother might say, "I wish all these slow pokes would..."