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An explanation from Barry Pollard at Digital Phablet breaks down why Google Search Console might flag an LCP (Largest Contentful Paint) as poor, even when individual URLs appear to have good LCP scores. Barry notes that this discrepancy stems from differences in how Core Web Vitals are measured in the report versus the Chrome User Experience Report (CrUX).
CrUX assesses real user data based on actual page loads, noting performance across a broad sample. The key metric, the 75th percentile, indicates that 75% of page views load within a certain timeframe. This method tends to favor highly cached or popular pages that load faster due to caching at various levels like CDN edge nodes or browser caches. These popular pages often dominate the data because they have higher traffic, biasing the overall performance score positively.
Meanwhile, Google Search Console evaluates specific URLs, often representative of the most visited pages, but it also considers individual page load times, including those with fewer visits or less cache efficiency. This means that even if most URLs perform well, a few slow-loading pages can drag down the overall website performance perception.
To analyze this further, Pollard suggests testing uncached versions of pages by appending a randomized URL parameter and rerunning performance tests. This process helps reveal the true load times for pages without cache advantages. If uncached versions take significantly longer to load, it indicates cache misses are hiding underlying performance issues that need addressing.
Since cache size and efficiency are limited, optimizing uncached load times becomes crucial, especially for pages with low visitation. Enhancing server response times, reducing code bloat, and leveraging techniques such as ignoring certain URL parameters (like UTM tags) in CDN caching configurations can help. There’s also an emerging standard allowing websites to specify which URL parameters can be ignored, helping maintain cache effectiveness despite dynamic or tagged URL variations.
Ultimately, understanding these nuances enables website owners to better interpret GSC reports and take targeted steps to improve performance, especially for critical pages that drive traffic and conversions.
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