Google says only a small percentage of queries will be affected and slow pages may still rank highly if they have high quality, relevant content.
The “Speed Update,” as we’re calling it, will only affect pages that deliver the slowest experience to users and will only affect a small percentage of queries. It applies the same standard to all pages, regardless of the technology used to build the page. The intent of the search query is still a very strong signal, so a slow page may still rank highly if it has great, relevant content.
Source: Google blog
As the change comes into effect in July, webmasters have time to make sure their site performs well.
Google recommends using a number of their tools to check and evaluate page performance:
Lighthouse audits the quality of web pages (including performance, accessibility and more)
Page Speed Insights indicates how well a page performs on the Chrome UX Report and makes improvement suggestions
Chrome User Experience Report provides experience metrics for how real-world Chrome users experience popular destinations on the web.
See Google’s blog post for full details.
