In his writing style, CS Lewis was essentially a sprinter/ short-middle-distance dasher (but Tolkien was built for marathons)

It is well known that CS Lewis wrote quickly, and revised very little.

But there are limits to what can coherently be achieved (with full and characteristic style) using this kind of writing method.

Lewis produced scores of first rate essays - done in a few hours each; and their coherence relies upon their being completed in a single burst of inspiration. But this does not scale-up indefinitely. Lewis was able to write up to the length of a short novel in this way - but when the piece didn't come out 'right first time' then he was never able to make the book cohere.

With the Narnia stories - those that were done in a draft have an effortless cohesion, while those that gave him some trouble - Prince Caspian and The Magician's Nephew - lack that spontaneity and fluidity.

That Hideous Strength is an excellent work, perhaps Lewis's best? - but it is a sum of rather distinct parts: it does not cohere well, it feels somewhat 'cobbled-together'; because it was considerably too long for Lewis's Mad Dash method.

The Screwtape Letters are similar; the book is a wonder, and I love it - but it is a loose collection of essays, not a unified whole.

It is noteworthy that the one book that Lewis really loathed writing (although it is very good!) was his contribution (on sixteenth century, non-dramatic works) to The Oxford History of English Literature. Like nearly-all academic texts, this is more like a mosaic than a thesis - and runs at about 700 dense pages. This was such a chore that he typically referred to his working on it as Oh Hell! (from its initials OHEL).

As a clincher, Lewis's most well-integrated long fiction is Till We Have Faces (making a paperback of about 350 pages), and this was written over an extended period. But TWHF is essentially a collaboration between Lewis and his wife - Joy Davidman, who apparently did a great deal of detailed editing work on the manuscript. Consequently, the book has a non-Lewis style, and reads as if by a different writer (which it was).

So, it seems that Lewis's strength was also his limitation. Because he wrote quickly and with concentration - he was very prolific (and indeed his letters, of which he wrote many per day, are of an extremely high, publishable, standard) - but when he could not finish a book satisfactorily in a single burst of rapid writing extending over not-many-weeks maximum; he was never able to achieve the spontaneity of style and effortless integration characteristic of his shorter works.

Lewis wrote and published far, far more good stuff than JRR Tolkien; but he never could have written a book of the length, complexity and excellence even of The Hobbit - never mind the Lord of the Rings...

Tolkien could 'hold' a work in his mind for months, years, decades... but the timescale Lewis was comfortable with was more on the level of hours, days or weeks - and then he wanted to move-on to some other project.