Martin Splitt, in a JavaScript SEO Office Hours video answered a question on improving Core Web Vitals score by blocking countries with slow Internet. The idea is that blocking those site visitors will prevent Google from incorporating those slow Core Web vitals metrics from being used to calculate the final score.
Core Web Vitals Scores
Core Web Vitals are a set of page experience metrics that will become ranking factors in 2021. There are two kinds of Core Web Vitals scores, Field Metrics and Lab Measurements.
Field metrics are actual scores derived from visitors to a site. Lab data are scores generated from a simulated visit through various tools offered by Google.