How much does site speed really affect SEO?
Two extreme opinions circulate about speed and SEO: that it does not matter at all, and that it is the only thing that matters. Both are wrong. Speed plays a limited, threshold-based role in ranking, while its largest effect appears somewhere else entirely, in whether the user stays on the page.
Speed is a ranking factor, but how
Google has counted page experience signals among its ranking inputs for years. That signal does not behave like a sliding scale, though. It will not lift an irrelevant page to the top because it loads quickly, but it can be decisive between two pages of comparable relevance.
The practical conclusion: speed will not rescue weak content, but it can block good content. A page sitting below the threshold is not facing an invisible penalty so much as a problem with the user's patience.
What the three metrics measure
LCP measures when the largest visual element of the page appears. The threshold considered good is 2.5 seconds. On corporate sites that element is usually the hero image or the main heading, so improving LCP often comes down to a single file.
INP measures the delay between a user interaction and the interface responding; the threshold is 200 milliseconds. A site that freezes for a second when the menu is tapped loses here even if its load time is excellent. CLS measures layout shift, with a threshold of 0.1. A button that jumps during loading is not merely annoying, it produces mistaken clicks.
The real impact is on conversion
The most measurable consequence of speed is not ranking but abandonment. On a slow page the user hits the back button, and the cost of that behaviour is direct: the form is not filled in, the quote is not requested, the document is not downloaded.
On enterprise projects this effect doubles on mobile. A site that looks fine on a desktop can be an entirely different experience on a mid-range phone over a slow connection. Testing on an average desktop browser is the easiest way not to see the problem.
The four most common causes
First, images. A single unoptimised hero image often accounts for more than half the total weight of a page. Modern formats and variants generated at the size actually used deliver the fastest win here. Second, third-party scripts: analytics, chat widgets, heatmaps and advertising tags add up, and how many times larger they are than your own code is usually a number nobody has checked.
Third, web fonts. Text staying invisible until a font file has downloaded delays LCP directly. Fourth, server response time: if the first byte takes half a second to arrive, no amount of front-end tuning can make up the difference.
Do not confuse lab data with field data
Laboratory tests simulate particular device and connection conditions. Field data comes from real visits. Core Web Vitals use the 75th percentile of observations, rather than a simple average.
The right method uses both: field data tells you which metric is failing, the lab tool shows you why. Chasing a perfect lab score on its own means spending weeks fixing a problem no real user ever experienced.