Core Web Vitals measure three user-experience metrics that Google considers essential for page quality. Largest Contentful Paint (LCP) tracks how long it takes for the main content element to render—ideally under 2.5 seconds. First Input Delay (FID) measures the time from a user's first interaction to when the browser can respond—target is under 100 milliseconds. Cumulative Layout Shift (CLS) quantifies unexpected layout shifts that cause buttons or text to jump around—aim for a score below 0.1. These aren't vanity metrics. Google confirmed in 2021 that Core Web Vitals are ranking factors, though their weight is modest compared to relevance and backlinks. A site with poor vitals won't tank if the content is strong, but when two pages are equally relevant, the faster one gets the edge. We've seen this matter most in competitive commercial queries where dozens of sites target the same keywords. Note that FID is being replaced by Interaction to Next Paint (INP) in March 2024, which measures responsiveness across the entire page lifecycle instead of just the first click. INP has a threshold of 200 milliseconds, and it's stricter—many sites passing FID fail INP because third-party scripts delay interactions after load. Core Web Vitals are measured in the field using real Chrome users through the Chrome User Experience Report (CrUX), which means your actual visitors' devices and connections determine your scores. A fast test on your MacBook Pro means nothing if 60% of your traffic uses mid-tier Android phones on LTE. Google Search Console shows your CrUX data broken down by mobile and desktop. At Ottawa SEO, we audit vitals early in technical reviews because fixes often require dev work—lazy-loading images for LCP, deferring JavaScript for FID/INP, setting explicit dimensions on media for CLS. Hosting matters too. We've migrated clients from shared hosting to optimized VPS setups and seen LCP drop from 4.2 seconds to 1.8 seconds without touching code. For most sites, LCP is the hardest to fix and CLS the easiest, but your mileage varies based on your CMS and theme bloat.