The American-English phrase slower than the second coming (of Christ, or of the Savior) means extremely slow.
The reference is to the Second Coming of Christ, which, in Christian theology, designates the prophesied return of Christ to Earth at the Last Judgement.
The earliest occurrence of the phrase that I have found—from the Weekly Arizona Miner (Prescott, Arizona) of 13th March 1874—is a jocular variant alluding to Montezuma II (1466-1520), the last ruler (1502-1520) of the Aztec empire in Mexico:
No other living thing can go so slow as a boy on an errand.—[Exchange.
Nonsence. A “religious” Indian agent going out of office is slower than a boy on an errand; yes, slower than the second coming of—Montezuma.
The second-earliest occurrences of slower than the second coming (of Christ, or of the Savior) that I have found are from two letters from the Philippines, written each by a U.S. soldier who was part of the contingent sent there during the armed conflict (4th February 1899 – 2nd July 1902) between the First Philippine Republic and the United States:
1: From a letter that “a former Kansas City boy”, whose name was withheld, wrote to a friend—letter published in the Ottawa Journal (Ottawa, Kansas) of 28th September 1899:
The only draft animals used here are the water buffalos, and they are slower than the second coming of the Savior.
2: From a letter that one Louis M. Wilson wrote to his family in Kansas on 29th April 1900—letter published in The McCracken Enterprise (McCracken, Kansas) of 6th July 1900:
There are some very fine churches here taking into consideration their tools and advantages. The churches are built of lime stone and dressed by the working Bolo 1, and magnesia is their lime, and it requires almost an age to build one, for the natives are slower than the second coming of Christ.
1 Here, apparently, bolo is the word designating a kind of cutlass used in the Philippine Islands for agricultural and domestic work and as a fighting weapon. If that is the case, this occurrence predates the earliest use of bolo recorded in the Oxford English Dictionary (2nd edition, 1989).
The phrase slower than the second coming (of Christ, or of the Savior) then occurs in a letter written by one John Jeems Jansen, “Financial Secretary 47”, published in Stove Mounters’ Journal (166 Concord Avenue, Detroit, Michigan: Published Monthly by the Stove Mounters’ International Union of North America) of November 1902:
The new appearance of the Journal is very unique, and allow us to compliment the designer. We trust that in the near future the members will pay a little more attention to it and send in a few lines every once in a while.
No. 47 received a ﬁne large bottle of the best brand of tobasco sauce what is, and intends to distribute some of it through the Journal to some of the corresponding secretaries. What is the matter with them? Some of them are slower than the second coming of Christ and even worse than half-frozen molasses. You write them a letter in June, and follow it up with another in August, and yet no reply; it makes one feel like kicking himself.
The phrase occurs in an article about George E. Q. Johnson (1874-1949), who, as United States Attorney for the Northern District of Illinois from 1927 to 1932, famously convicted the U.S. gangster Al Capone (1899-1947) for tax evasion—article published in the Chicago Sunday Tribune (Chicago, Illinois) of 21st December 1930:
George E. Q. Johnson is slow. That, probably, is why he is inexorably sure when he finally swings into action. Once Senator Deneen 2 said of him, “Yes, George E. Q. is slower than the Second Coming, but he grinds and grinds and grinds all the time.”
2 Charles Samuel Deneen (1863-1940), United States Senator from Illinois from 1925 to 1931
The phrase then occurs in the account of the trial of twenty-five men charged with liquor smuggling—account published in The Times-Picayune (New Orleans, Louisiana) of 3rd May 1933:
The principal part of Tuesday morning’s session was taken up by Mrs. Elizabeth Smith Friedman, Washington, D. C., cryptanalyst, who […] interpreted numerous messages which were alleged to have been sent from a powerful wireless station here to Belize, British Honduras, and to “mother ships” loaded with liquors in the Gulf of Mexico.
The messages dealt with the operations of rum ships and arrangements for contact of the ships by speedy rum-running boats, at points alleged to have been designated by members of the conspiracy for anchorage of the mother ships […].
One message purportedly from an anchored rum ship was read, “Shore boat slower than the second coming. Do not drink all of the beer. We are all thirsty. If the boat does not show up we will return to Belize.”
The Scottish poet and nationalist Hugh MacDiarmid (Christopher Murray Grieve – 1892–1978) used the phrase in from The War with England 3—as published in Collected Poems of Hugh MacDiarmid (New York: The Macmillan Company, 1967):
The social scene could be little
But confusion and loss to me,
And Scotland, better than all your towns
Was a bed of moss to me.
I had to lie on the hills and watch
The founts that to keep their tryst
Had found their way through the wards of the rock
Slower than the second coming of Christ
To know how my task was priced.
3 According to the Scottish poet and academic Alan Riach (born 1957) in Hugh MacDiarmid in Context, Hugh MacDiarmid composed this poem during his retreat in the Shetland Islands from 1933 to 1942.