I have had a few workshops on speed reading at my university, and what those guys said about subvocalization was something along the lines of:
We learn to read when we are about 6 years old, how do we learn this? By sitting in a classroom and reading out aloud a simple text, with the entire class at once. In this stage we start to connect reading with subvocalization, when we learn to read better the teacher tells us to 'read in our heads'. Meaning that you are still sub vocalizing. This is a limiting factor as one can (presumably) only speak around a maximum of 500-600 words per minute. Coincidentally, this is the maximum speed for which we can read with a high comprehension according to skeptoid.
I personally never thought much of the guys giving the speedreading workshops, they seemed to be acting and came across like secondhand car dealers. Their claims of comprehension with over 1500 words / minute just sound insane. The techniques however are not all nonsense, and I do believe that I have learned to increase my reading speed from +/- 300 to around 500 words per minute.
The most important lesson they thought was 'guiding your eyes'. Our eyes are not that good in following a straight line by themselves, just try to stare straightforward, and make a perfect circle with your eyes: it will more likely be a pentagon or something. When we let our eyes follow our finger drawing a circle however, it is a lot easier. Applying this to reading, simply means using your finger or pen to follow each sentence on the paper. I find it a lot faster to read that way, but I still let myself subvocalize the sentences, for comprehension. Doing this on a 500-600 words/minute rate is a lot more exhausting though, and I only use it for academic articles or textbooks.