The more radiant heat that can be directly targeted onto your body the faster you will sweat since the Infrared Heat is absorbed by the moisture in your skin, drawn in deeper through conduction to the muscles to induce a deep heavy sweat.
The majority of Infrared Saunas on the market today do not have heaters installed on the front wall. In addition to this, what you see in most brands is the side wall heaters stopping short at the front of the seat. Therefore no radiant heat is projected on to the front of your body.