You have a list of opportunities* that may improve the speed metrics of your website. For example Total Blocking Time (TBT) is one metric that heavily impacts the experience of a visitor.
You've run lighthouse (or any other speed testing tool), which tells you "Estimated Savings" for each of these opportunities.
You do not have enough time to fix every opportunity, so you need to narrow it down to some.
In Chrome browser, how would you measure the real life savings of a specific opportunity, in a production build?
For example, if I remove script x, how do I measure how much that impacted the Total Blocking Time, with a before and after metric?
Basically, so I can say "I took this action, and this is the direct impact of that action on site speed".
Note: I asked this same question on stack overflow, but I think it might be a little too broad and methodology oriented for there, which is why I've come here.