The term "absorbency rate" refers to how quickly a material can soak up or take in a liquid or moisture. It describes how fast a substance can absorb or hold onto liquid. Full definition
Does the difference in weight and absorbency rate really make a difference? (foodiefiasco.com)
The other protein being casein, however, whey is the one widely accepted and used as compared to casein due to its rapid absorbency rate and also because it is less likely to cause digestive problems. (blog.oziva.in)
They also like the absorbency rate, stating that they are great for moms who have extreme let down. (omaby.com)