Google Search Advocate John Mueller highlights a new case study explaining why certain Largest Contentful Paint improvements often fail to produce results. In a layout that varies by store, the browser may focus on the wrong element, causing all subsequent optimizations to target something that was never the LCP in the first place.
The case study was published on web.dev on June 24, and it details a year of Core Web Vitals work at the ecommerce platform Nuvemshop. Initially, the team suspected image weight or server latency as the main issues. However, their analysis pointed them toward how the browser selected the LCP element. After optimizing element selection, image priority, lazy loading, and edge caching, Nuvemshop reports that the share of its stores with good LCP scores increased from 57% to 96%.
How the Wrong Element Gets Picked
On Nuvemshop, merchants can organize their homepage sections in any order they prefer, resulting in carousels, banners, and product grids appearing in various positions across different themes. According to the case study, carousels were present on 85% of storefronts, and this is where the measurement encountered an issue.
The issue stemmed from CSS transitions on carousels and banners. These transitions delayed when elements became visible to the browser’s LCP detection. While shoppers saw the carousel load first, the browser occasionally identified a banner further down as the LCP element because its visibility was not delayed by a transition. Consequently, according to Nuvemshop, the previous optimization efforts focused on elements that were never the LCP.
The investigation identified three main causes. Transitions were pushing detection to the wrong frame, lazy loading was delaying images above the fold that needed to load immediately, and the critical images lacked priority signals. Additionally, measurements originated from both category and product pages, not only homepages, so the solution had to be effective across different page types.
The Three LCP Fixes
The case study includes a fourth change, edge caching to reduce latency. However, the core element-detection adjustments involved three modifications in how the top of the page renders and loads. Nuvemshop implemented all three changes across its main themes and page types. These adjustments are straightforward, following common web performance best practices.
The team removed CSS transitions from the top sections so they display instantly, enabling the browser to recognize them as LCP candidates without delay.
They also pulled loading="lazy" off the first image in those top sections. web.dev’s own guidance is blunt on this, warning against lazy-loading the LCP image because it always adds load delay.
The last piece was fetchpriority="high" on the likely LCP image, which tells the browser’s preload scanner to grab it sooner. Nuvemshop says it wrapped that in validation logic so the signal fires only where the element could really be…
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