Molasses flows slowly, but it flows even more slowly when a tree is cold or frozen (such as in the winter month of January). The expression “slow as (cold) molasses” has been cited in print since at least 1854. “Slow as molasses in January” (frequently written as slower than molasses in January") has been cited in print since 1859.
Wiktionary: slow as molasses in January
slow as molasses
slow as molasses in January (not comparable)
1.(simile) Extremely slow
Idiom Definitions for ‘Slower than molasses going uphill in January’
To move extremely slowly. Molasses drips slowly anyway but add January cold and gravity, dripping uphill would be an impossibility, thereby making the molasses move very slowly indeed!
July 1854, North American Review, pg. 150:
“Yes,” was the drawling reply, “but he is—as—slow—as—cold molasses.”
9 December 1854, New York (NY) Times, pg. 8:
“Yes; but he is as slow as cold molasses.”
13 August 1859, Oconto (WI) Pioneer, pg. 3, col. 3:
OLD BOY BOOTH—DOUGLAS AND DEMOCRACY.
MILWAUKEE, Aug. 4, 1859.
DEAR PIONEER:—The Booth trial is as slow as molasses in January.
15 November 1864, The Canada Farmer, pg. 346, col. 2:
Along he drew himself slower than molasses in cold weather, and let the rest of the moving animals pass and repass him.
9 April 1870, Harper’s Weekly, pg. 238:
You’re slow as molasses in the winter time.
28 December 1872, Chicago (IL) Daily Tribune, “The Credit Mobilier Investigation,” pg. 4:
Most of them had the matter under advisement for seven or eight months before they could satisfy their consciences as to the moral bearing of the transaction, showing that the average Congressional perception of right and wrong is much slower than molasses in January.
16 January 1879, Lowell (MA) Daily Citizen and News, pg. 4:
There is believed to be only one thing slower than molasses in January, and that is a lady making room for another lady in a street car.—Chicago Herald.
27 April 1889, Harper’s Weekly, pg. 330:
Everybody knew he was slower than molasses in January, like those Pennsylvania Dutch.