"How important are the image height/width attributes and would you expect a best practice site to have them included ?"

This is not the most important SEO thing in the world BUT according to your 2nd question

"I hear not having them can slow down a page load time is that correct ?"

That`s the point! The question related to this issue is how relevant the whole thing might be?

Modern browsers and broadband connections seem to make this insignificant but just in case there are some visitors which are not using the right settings they might get pictures unscaled and your whole site will be shown non-responsive... by the way, use responsive designs if you can to avoid that...

Ive always been told to use these parameters . even if you dont need it it ensures that your code is a little bit more perfect