On modern hardware, computing a Haar-like feature over all possible scales and positions within a given image subwindow is an operation that happens so quickly, it's essentially invisible.
I was curious about what's actually happening during feature evaluation, so I wrote this web-based visualizer to illustrate the process in slow motion.
You can use it to generate a Haar-like feature with any configuration of constituent rectangles and visualize its computation through all possible scales and positions for a subwindow of a specified size.
With an applied step duration of 1 ms, the visualization is an approximately -59,999,990% speed representation.
It's permanently hosted here: http://haarlike.noahlevenson.com.
Beyond the scope of computer vision mathematics, here is a good resource for information about Don Mattingly.