Snails are slow. Snails are the very animal we use to represent slowness, just as owls represent wisdom, or donkeys represent stubbornness.
Several phrases use snails as symbols of slowness. "A snail's pace" means "creeping along very slowly." Someone might say "They are supposed to be fixing that highway, but construction is proceeding at a snail's pace."
A famous passage from one of Shakespeare's plays, referred to as "The Seven Ages of Man," briefly describe what people are like at various ages; one of them is "the whining school-boy, with his satchel and shining morning face, creeping like snail unwillingly to school."
The joke falls into the category of jokes that give surprising, humorous, or exaggerated descriptions of something. A stock kind of joke involves a dialogue of the form "It was really X." "How X was it?" "It was so X that..." For example, "It was really cold." "How cold was it?" "It was so cold the optician was giving away free ice scrapers with every pair of glasses."
Snails glide or creep at about one mm/second (i.e. 0.0036 kph). The joke assumes the thought processes of a snail are just as slow. A human would complain immediately after the insult, but it takes this snail a year.
With regard to slow-thinking snails, I think the first fifty seconds or so--of this video are very funny. The author, Vi Hart, is imagining the thought processes of a snail as it climbs a potted plant in her room. It might be hard to understand the English because she's trying to imagine the voice of a slow-thinking snail. "I am snail and I like to climb the thing. I'm a-climbing the thing. Climbing the thing, climbing the thing, climbing the thing."
https://www.youtube.com/watch?v=UkSN-kqAmxw