- darken the text, low contrast destroys eyesight.
- make the text bigger for god sake
- use a serif font for the main text,really,you can keep the headlines sans.
- your website has a big whitespace/negative space balance problem.
I mean,i dont understand designers today. Do they care about readability? or that a site look fancy on their shiny new mac ?
As for performance problems i would suggest moving to a static site generator, a doc is the perfect use case.