Styled HTML is better for SEO because usability is a search engine ranking factor. Unstyled documents are ugly and hard to use. When users land on such pages from search, they will be unhappy. Many of them will us the back button and find other sites to use. Google will notice this behavior and worsen rankings for pages that users don't like.
On the other hand, performance is also a search engine ranking factor. Improving site performance can help rankings. However, for performance improvements to help SEO, they usually have to be on the order of seconds. Spending a couple hundred milliseconds to apply styles to pages isn't going to make a huge difference to SEO in terms of performance but will make a huge difference in terms of usabilty.