The problem is the problems are hard to measure. How do you know that "Joe" bought from someone else because the competitors website just "felt better". We know from UX research how speed affects how things feel. We know that customers will go elsewhere at times. We also know not everyone who visits your site will buy. I know statisticians have tools to measure the loss, but I'm not able to tell you how they do it or how accurate they are (they can give you a range).