Charles LaRue said:
By the Valar, The Silmarillion is the most obtuse book ever written, with the possible exception of Paradise Lost.
The Silmarillion was not written. It was compiled. It is a reference work, not a narrative. It's like saying "The phone book was the most boring book I ever read. It was just names and numbers cover to cover."
I found that some aspects of The Silmarillion help add meaning and depth to the content of The Lord of Rings. One of the things to appreciate when reading LotR is the sense of the weight of history a lot of the characters are carrying around with them, Aragorn being the foremost. Look at how the Elves are portrayed, as a race suffering from what is essentially terminal sadness. Why? The Hobbit and The Lord of the Rings don't tell you. The Silmarillion does.
Other parts of The Silmarillion are Tolkien's own reference notes about geography and such that were never intended to be of intrinsic narrative worth. And thus they are slog to read through.