Reading in my head certainly takes the same amount of time as reading out loud (occasionally with different voices for characters, as somebody else said).
If I read without doing that it’s a lot quicker but it doesn’t go in and I have to re-read it. My mind starts chatting away about something else rather than concentrating on the book.
Mostly it’s my own voice for comments 🙂, maybe a slight inflection. I don’t usually go overboard on the voices unless it’s somebody I know, or occasionally characters in books.