How can I fix shifting elements on my site due to custom CSS?

Store Design
1

Hi, my website displays fine but when it loads the user can see everything shifting into place from my custom changes by CSS and I’d like to know if there’d be a way to fix this. It’s difficult to explain what I mean but visit my website www.stagtis.com and refresh and watch the header and also my testimonial from Loox. It’s as if they load slower than the rest of the store. Thank you so much