Click here to see an Android Police article from 05/03/2022 called "How to fix a slow Chromebook with chromeOS Diagnostics" that tells you how to find and run the App, then how to interpret the results.
Click here to see official Support Pages for Chromebook Help.
Click here to see an article by ChromeStory from Nov 2019 called "How and Where to Get Help with Chromebook" that gives more good advice.
Click here to see a support page with an Installation Guide and a list of Certified Models.
[being rolled into chromeOS Flex in 2022]
Click here to visit this useful online resource at the Support Centre.