It's complicated! (I'm not an expert, just a curious person who likes to read about reading strategies.) People who subvocalize are sometimes trained to stop as a strategy for reading faster. However, it's important to note subvocalization is a matter of degree: it's not on or off, but comes in many levels of intensity. It seems to affect certain types of comprehension and memory retention, particularly increasing comprehension at a high level and retention into short-term memory. It's not clear it's such a bad thing, but maybe being able to turn it off (or up and down) might also be a good skill to have?