different between plodder vs slowcoach

English

From Middle English plodder, equivalent to plod +? -er.

plodder (plural plodders)

One who plods.
- 1842, Robert Browning, The Pied Piper of Hamelin, London: Frederick Warne, 1888, p. 18,^[1]
  Brown rats, black rats, grey rats, tawny rats,
  Grave old plodders, gay young friskers […]
A person who works slowly, making a great effort with little result; a person who studies laboriously.
- c. 1594, William Shakespeare, Love’s Labour’s Lost, Act I, Scene 1,^[2]
  Study is like the heaven’s glorious sun
  That will not be deep-search’d with saucy looks:
  Small have continual plodders ever won
  Save base authority from others' books
- 1899, Pansy (pseudonym of Isabella Macdonald Alden), Three People, Boston: Lothrop, Lee & Shepard, Chapter 21, p. 271,^[3]
  What an indefatigable plodder you are to get those papers ready so soon, and an unmerciful man besides to make me go over them to-night.
- 1939, Robert James Manion, Life is an Adventure, Toronto: Ryerson Press, Part Two, II, p. 43,^[4]
  Throughout my life […] I have been fortified in the conclusion that it is much more important for a young man to be a worker, even though not brilliant, than to be brilliant and not a worker. As one looks back at some companions who attended lectures, and follows the records of their lives, one is strengthened in this belief because the facts show that the steady plodders have gone further than have some of those brilliant lads of other days.

slow +? coach

slowcoach (plural slowcoaches)

(informal, Britain) A person, especially a child, who moves slowly.
Synonyms: plodder, sluggard, (North America) slowpoke
Hey, you slowcoaches at the back! Get a move on!
- 1911, Chesterton, The Innocence of Father Brown, The Blue Cross:
  They were both very quiet, respectable people; one of them paid the bill and went out; the other, who seemed a slower coach altogether, was some minutes longer getting his things together.
(informal, Britain) A person who is slow on the uptake; one who does not comprehend new ideas quickly.