I want to make an all-in-one quick reference page that transcludes (approx.) 80 sections from 40 different pages, but I don't know if so many transclusions will cause problems, and if it's just better to write everything out on the comprehensive page instead of transcluding.
Does putting too many section transclusions significantly slow down a page/take up excessive resources, and if so, what's a better practice?
I am not an expert but, based on my understanding of how MediaWiki works, sort of. It will slow down page rendering and require a relatively large amount of resources. However, that should only be the case when rendering the page. Once the page is rendered, the result is saved (a.k.a. cached) for a while before MediaWiki tries to re-render the page. Those who view the page between renderings will be quickly served the cached version so there probably would be little or no difference to them. However, those who happen to trigger a re-rendering may see a bit of delay.