We like to think of these as medieval English, but they're not. They are Elizabethan English, Renaissance English, Shakespeare's English, a culture more advanced and technological than that of Gondor. The English of the Middle Ages would have been unintelligible to us, Old and Middle English were basically Anglo-Saxon--like reading Beowulf in the original. It wasn't till after the 13th century, due to the Norman French influence and Chaucer's wide acceptance as a writer and poet, that anything resembling what we speak today became common in England. Chaucer spoke spoke and wrote the Common Tongue of the London area, while French was the language of the Court and Latin that of Scholarship. And of course there were regional and ethnic languages that continued to be spoken for centuries after, the Gaelic tongues of Scotland and Wales, Cornish, and so on.

After the Norman Conquest (1066), Anglo-Saxon was spoken by the peasantry, and the aristocracy spoke French. These two eventually merged into the English we know today. Of course, this was Tolkien's area of expertise, he would have known this. Why he shifted to this stylized speech was no doubt planned, but I can only speculate why.

As for the dragging, depressing pace of Book Four, I can only guess that it was the influence of Gollum and the increasing isolation and fatigue of the Hobbits--the burden of the Ring. Its not fun reading, but perhaps it is necessary. The film also captures this very well, I wanted to strangle Gollum myself long before it was over.

I will comment more on this after I read that part.