> it'll ding you for loading external files

The Opportunities and Diagnostics sections don't contribute to your Performance score. Your overall Performance score is a weighted average of the Performance metrics [1]. The Opportunities and Diagnostics sections are just potential ideas on changes that may help you. It's up to you to decide what's best for your site.

[1] https://web.dev/performance-scoring/

Disclosure: I work on Lighthouse and web.dev

The Opportunities and Diagnostics sections may not, but your Performance score is affected by loading additional external files.

I've learned a ton for web perf stuff from the devtools and Lighthouse docs, so thanks for your work on those. :)