The answer is that it depends on the person and further on the type of yoga and reason for doing it.
For many people a slower form of yoga can bring a great sense of peacefulness and relaxation. The focus on deep breathing with gentle and balancing postures can bring about a release of endorphins, muscle tension, and mental focus.
On the other end for some people if they are "trying to hard" or doing some of the faster or more difficult forms of yoga the result can be less peaceful and may even cause a sense of aggravation and pain - often both able to be worked through with time but for that immediate sense of peace to the mind may not work.
I usually suggest the combination of deep breathing, meditation, and some form of the slower stretching/movement as the first person suggest Tai Chi is such a form. Additionally anything that will bring about a higher level of mindfulness may create that peace as well as something that lets you "slip away" such as a relaxing massage.
Ultimately again the answer comes back to you and whether you want, believe, and discover that it works for you!
