Google PageSpeed Insights is the one and a page speed is just for loading the viewport, i.e. initial load. There are others but they give more information to help determine the output of GPI.
So if you had an image that is lazy loaded, 5 pages down that is 100MB it would not affect the speed unless the browser decides it wants to download it as part of the viewport.
If you had 100MB CSS or HTML or FONTS it would affect the speed.
If that answers your question can you please tick solution and show forum love by ticking the heart. Helps keep forum tidy.
PS For others reading, I am a consultant who specialises in Webflow performance scores. PM if you are interested.