Yes, it really is supposed to be played that fast! Here’s what it’s supposed to sound like, if you don’t know the song, or copy and paste this: http://www.christmas-carol-music.org/SATB/DingDongMerrily.mid into your browser, if the link doesn't work. The original was a French dance tune without words, called “le branle de l'Official”. It was first published in the late 1580s by French cleric and dance historian Jehan Tabourot, under the pen name Thoinot Arbeau, which is an anagram of his real name. No one now knows if he composed the tune, or merely collected it. I have not been able to locate any French lyrics, either ancient or even a modern translation from the English. Such a translation would have to be decidedly modern, as the English lyrics were not written until 1924!
The lyrics were written by George Ratliffe Woodward, who adapted the somewhat worldly dance tune for use as a Christmas carol. Their archaic flavor stems from the author’s delight in archaic verse and language, and references to his hobby of church-bell ringing. His deliberate archaisms include the words sungen, swungen, matins, evetime and Io, io, io, as well as his deliberate use of the Latin chorus: gloria, hosanna in excelsis. This is a quotation from the Latin Bible, and means, “Glory and praise in the highest,” the phrase which the people shouted to Jesus on his triumphal entry into Jerusalem, in recognition of his Messiahship. It is often mispronounced hose Anna in excel sis. In reality, it has nothing to do with spraying Anna with water or exceeding your sister, and should be pronounced hoe sauna e’en ex shell cease (e’en though it has nothing to do with gardening in a sauna or stopping an artillery barrage!) Io, io, io is pronounced, "ee-oh, ee-oh, ee-oh," NOT "eye-oh, eye-oh, eye-oh,." At speed, it sounds rather like a piglet squealing. Oh well!
I first encountered this delightful tune in the movie, "Little Women," and was devastated to learn that Jo and the girls could not really have sung it, as the lyrics would not be written for more than sixty years! It’s still my favorite version of the song. The harmonies (also from 1924) were composed by Charles Wood. In any event, the tune is in the public domain, so there’s no problem with publishing it.
This is really not a hard song to play-- slowly. The difficulty comes when you try to play it at speed. Don’t try this until you have the song completely memorized! Even then, start at half speed or slower and work up gradually, or you’ll never get it right. This may take some time, so start working on it early. Remember, it’s much better to play it too slowly than to mess up the rhythm. If you play too slow, the audience may think you’re doing it on purpose, if they even notice. After all, this is a guitar solo; there’s no one singing or dancing! If you lose the rhythm, though, it’ll show.