I found this interesting article on Career Builder. It confirms what I say to my students over and over again. Don’t speak so fast. Not only will your accent be stronger, but you might be perceived as insecure or not careful about details. There’s nothing wrong with speaking slowly unless your voice is monotonous. This is easy to fix when you learn the rules of word stress.
If you already speak quickly in your native language, your accent in English will be so much more difficult to understand if you don’t slow down. You will be likely to swallow a lot of consonants.
Sometimes I record my students while they are speaking in class. Then when I play it back to them, even they don’t understand what they said. Try recording your voice and then analyze how you sound. You might be shocked by what you hear.
This article also discusses other types of voices and speaking styles and how they are percieved, particularly during job interviews.
There’s a good book on this topic called: “Change Your Voice, Change Your Life” by Milton Cooper.