Start with Measure first, and keep to its order: the server's time to first byte, then the browser's waterfall, then the template. It tells you which half of this unit your page needs.
If the server is slow, the cause is almost always in the template rather than the platform. The three query-per-row shapes teaches you to recognise the usual one, Fetching once instead of per iteration shows how to fix each shape, and The cost of nesting covers component trees, including when to remodel the content instead of tuning the templates. If the browser is slow, Images, Bundle size and Fonts and third-party scripts each give measured numbers and a budget. Pages that are served from the cache matter less here than the ones that render on every request; the caching unit before this one says which those are.