I have this experience when I read and really get immersed. I have the feeling that a noticeable or persistent voice in my head is more of a symptom or signal of my distractedness than the cause.
Also, how did you measure your reading speed, is your comprehension good at the upper end, and did you do anything to purposefully increase the reading speed?
I measured it with the first two reading speed websites that came up on my google search here and here. My reading speed was also tested when I was 16, with very similar results.
My comprehension is very good (in the 80-90% range) at the upper end for anything I can visualize, or when the details of the sentence do not change the meaning. For example, my comprehension would be very high for an action scene in a novel, but when I have tried to read that quickly about any non-familiar technical topic, my comprehension has fallen to the 20-50% range.
I did ...
http://en.wikipedia.org/wiki/Subvocalization
I'm curious about how often or to what degree visitors to this site subvocalize as they read. I was originally interested in reducing subvocalizations as a way to increase reading speed, as the idea is mentioned in multiple pieces I've read about speed reading.
The Wikipedia entry seems to focus on subtle throat and muscle movements, but I'm more interested to know if you "say" or "hear" the words in your head as you read.
Since reading about subvocalization recently, I seem to notice that I "say/hear" what I'm reading quite frequently. I'm not sure if this is causal (in the way that the command "don't think of pink elephants" obliges you to do so), or if I just notice it more now, or both.
When I'm very engrossed in a book either I don't notice the subvocalizations or they stop happening, so seems that it could either be a cause or a symptom of distractedness.
In the comments, please describe your mental subvocalizations (or lack of them) and if they are related to how engrossed you are in the book. Any other comments relevant comments about speed reading or subvocalizations are welcome.