What makes the heart work harder from a 'thickness of blood' standpoint is your systemic vascular resisitence (SVR). SVR increases the afterload (resistence the heart is pumpimg against). As blood becomes more viscous (a higher HCT.) SVR increases which in turn forces the heart to work harder. This isnt that crucial because our bodies have the ability to auto-regulate SVR thus keeping the afterload of the heart within a certain range.