We've started tracking Shopify speed tool scores on about the last couple hundred sites we've optimized (speedboostr.com), the range we see is usually between 20 - 35 for active stores using apps, with the average being 23.
Optimization usually raises the score a little bit, but the score with that tool is largely based on the 3rd party apps you use so it's common to score low if you have store facing apps. It uses Google Lighthouse, the same general engine that Google PSI uses. To see how apps affect the score with that tool we did a case study: https://speedboostr.com/how-apps-affect-load-speed.
A 40 is a pretty good score with the Shopify speed tool if you have an engaging site.
• Creator of Order Automator (automation app to save you and your team time on admin tasks) • Shopify developer for +10 years • Contributor on the Speed Boostr blog • Guy who likes to help people