I'll agree that 3x to my ear sounds insane, but so did 2x a few years ago. I've definitely adapted to listening at higher speeds as I've got used to it. I can believe that some people are comfortable with it.
But you're right, sentence structure does play a part. I've listened to Treasure Island (Robert Louis Stevenson) and Excession (Iain M Banks) recently and both I had to drop to 2x because I couldn't follow at 2.25x. The latter because of the long ship names that sounded like sentences took too long for my brain to parse out and comprehended, and the quite technical language. The former because of the slightly old fashioned and uncommon language. On the other hand, with another recent book, Under The Skin (Michel Faber), I was totally comfortable at 2.25x. The narrator was nice and clear with no strong accent, and the language was modern and descriptive but not overly complex.
But you're right, sentence structure does play a part. I've listened to Treasure Island (Robert Louis Stevenson) and Excession (Iain M Banks) recently and both I had to drop to 2x because I couldn't follow at 2.25x. The latter because of the long ship names that sounded like sentences took too long for my brain to parse out and comprehended, and the quite technical language. The former because of the slightly old fashioned and uncommon language. On the other hand, with another recent book, Under The Skin (Michel Faber), I was totally comfortable at 2.25x. The narrator was nice and clear with no strong accent, and the language was modern and descriptive but not overly complex.