It depends what tool you’re using.
If you’re using the Shopify Analyzer, then usually a higher score results in a faster website because the optimization warnings are based on factors that contribute to actual load speed.
If you’re using Google PageSpeed Insights or the Shopify speed score (both are based off the same engine so have similar scoring), then the score is based more on emulation than real load speed. Some key optimizations like lazy loading or image optimization may raise the score while also improving load speed, but sometimes raising the score in an emulation based tool can decrease actual performance. To see how load speed compares to score with these tools, see Case study: Does Raising PSI Score Improve Page Speed?
To measure actual load speed, use WebPageTest.org. We recommend focusing on optimizing what’s important for Shopify sites first (see our services listed above for ideas), then if you want to dive into warnings in particular tools, we can start an analysis project to see if anything can be done to further improve performance + advise on solutions.