The key to this method is the formulas have already been converted to postfix notation and all the evaluator function has to do is peddle down the string evaluating the variables as you go and then applying the op code (+ - * /,etc.) to the variable. If you use an HP12c calculator you would be familiar with postfix notation. Formulas are generally pre-thought out and entered via key strokes. Once the operator exits that field the program would immediately format the formula into postfix noation ready for the processing program to select it and evaluate it. You have to think how something like this would be used. At a data entry level you don't have to worry about speed.