The Silmarillion is a slog.

Ive always believed that Tolkien should have turned it and into an epic like LOTR.
Yes! I actually really enjoyed The Silmarillion and it only took me three days to read (which is slow for me, as that size book would normally take me just a few hours or a day at most), but it has sooooo much in it! It's not really one story, but many and each one I think should have been explored in their own glorious epic instead of glossed over like an overview of history. It leaves me wanting more.
Actually................ no. One of the reasons LOTR was published in three volumes was the high cost of paper and typesetting at the time (paper because it still wasn't that long after WWII).
And Stanley Unwin thought that the target readers wouldn't be able to afford the price the book would have to be sold at if it was a single volume.
I will add that in my opinion what makes the Silmarillion a difficult read is that it is TOO SHORT, since many of the stories, as I mentioned much earlier, are not final drafts. Nothing is more boring than something that reads like an outline.
It's unfortunate that the final story, "The Voyage of Earendil and the War of Wrath" is among the least worked-up - it's an anticlimax reading that after "Beren and Luthein" and "Turin Turambar."
If one is traveling on a fascinating journey what is the use of racing through it? I read The Lord of the Rings in a few days, and when it was over, I wished it had lasted longer.

Some of his other works, like The Hobbit, Farmer Giles, Smith of Wootton Major, are actually quite short. I seem to remember reading Farmer Giles in less than a day and then immediately rereading it since I enjoyed it so much the first time.
Smith of Wooton Major is a fantastic little tale.