The fact that giving a presentation at the black board slows down the speed of presentation and helps the audience to digest the stuff is related to the fact that mathematicians use a language that has a high information density (formulas, diagrams) compared to other fields. Moreover the symbols and other structures used in that language CAN be written on a black board. This is different for a biologist say: using a black board for him/her can be very inefficient, because of the necessity to write down more or less complete sentences or long phrases. This is too much of slowing down. Information dense structures used in biology are pictures or visualizations of numerical data that are hard to create at the black board.