Would you also happen to be a fast reader? I can't seem to break the 120 barrier, and I'm wondering if the habit of mentally "saying" the words as I read them is a significant reason why I can't improve.
As dbarlett pointed out, the sub-vocalization is a big part of it, but once that's gone, there's still a long way to go to read quickly and effectively. I learned to read exceptionally fast, but had a very low retention rate of what I read (I use those techniques when typing), it's symbol recognition/word recognition, not real comprehension. I eventually slowed down my reading, but followging a rhythm (per-word rate) and process/think about/determine application for what I've read up to that point. In the end, reading quickly hadn't been as beneficial as being able to have a strong working knowledge of what I've read after only the first read through. However, I dislike reading fiction and don't prefer reading stories, an in depth whitepaper or textbook is my enjoyment reading, so learning as I read was more of a priority than just getting the high level view or enjoying the story construction.
[1] https://en.wikibooks.org/wiki/Speed_Reading#Subvocalization