But I remember when I first read LOTR as a young teen, my opinion was rather that the rest of the mammoth novel was a bit rushed. After so much detail, so deep a draught of Middle-earth as The Fellowship of the Ring offered, I expected more battles like Helm's Deep and the Siege of Minas Tirith, more delayed climaxes. I was shocked that this war that had simmered on the hob for so long and finally overboiled its pot should be completely finished in only a few months. Indeed, between the breaking of the Fellowship at Rauros and the fall of Sauron is only twenty-nine days. (The Fellowship spends a whole month in Lothlorien, doing, well, not much, and there isn't much to say about it. This adds to the sense of too-quick resolution.)
True, the breakneck pace after Rauros is attractive. It holds your interest. But C.S. Lewis pointed out that he and Tolkien were both veterans of the First World War, and there is a flavor (to his generation) of that war in the book. Yet that war was as long and heart-breaking a struggle in the mud and blood of war as can be imagined. With our focus tightly kept on the members of the Fellowship, we don't see as much of the struggle between Sauron and the West, and LOTR suffers for it, I think.
Would people stand for a Hobbit War and Peace? Probably not. And it is still a very great work. But whether it takes too long to get going or rushes toward its conclusion is very much a matter of opinion.