Is the Paintbrush Tool more hardware intensive than the Pencil Tool?

That would depend on how you use it. 

The brush toopl will always immediately apply a brush to the path you draw. And it will also respect the graphic tablet input (pressure etc). So if you use a complex brush and complex graphic tablet input, then the brush tool is more hardware intensive than the pencil tool, because the pencil tool just draws a line (which the brush tool does on top of everything else)