Tolkien nods - a rare, but significant, lapse in Lord of the Rings

I am delightedly reading aloud The Lord of the Rings at present, and yesterday I noticed a key short passage in which Tolkien's usual high standard of writing lapses - and realised I had been unconsciously aware of this defect for more than forty years.

I also think I can infer why this particular subject matter was done sub-optimally. 

Can you guess which passage I mean?

Answer is at the Notion Club Papers blog.