Experiments on the image audits in Lighthouse, the tool behind PageSpeed Insights: when ‘Properly size images’, ‘Serve images in modern formats’, ‘Efficiently encode images’ and the lazy-loading audits fire, how much the estimated savings match what a real conversion saves, and how much scores move between identical runs. Each guide builds fixture pages that cross an audit’s threshold on purpose and runs Lighthouse against them several times, so you can see where the lines really are before acting on a report.
Lighthouse LCP Request Discovery: What Each Hero Setup Gets
The insight passed only heroes with fetchpriority=high and flagged lazy and script-inserted ones, but simulated throttling reported the lazy hero 600 ms faster, the opposite of applied throttling.
Lighthouse Audits 3 min read
Does Lighthouse Catch Bad Alt Text? Ten Cases Tested
Lighthouse 13.5 failed only the image with no alt attribute. File names, the word image and alt text starting with Photo of all passed; redundant alt text was noted without affecting the score.
Lighthouse Audits 4 min read
Lighthouse ‘Avoid Enormous Network Payloads’: Where the Warning Starts
Lighthouse 13.5 warned from about 2.7 MB of transfer, with no extra penalty for heavier pages, and the performance score stayed at 75 from 1.6 to 13 MB while LCP reached 26 seconds.
Lighthouse Audits 3 min read
Which JPEGs Does Lighthouse Still Flag for Format or Compression?
Lighthouse 13.5 passed JPEGs up to quality 85 and flagged them from quality 90, plus PNG photos. A quality-80 JPEG passed although AVIF would have saved 54 %.
Lighthouse Audits 3 min read
When Lighthouse Flags Oversized Images, and How Accurate Its Savings Are
Lighthouse 13.5 flagged a photo once it was about 8 % wider than its slot needed, not at 5 %. Its estimated savings were 14 to 29 % higher than what a real resize saved.
Lighthouse Audits 3 min read
Lighthouse Simulated vs Applied Throttling on Image-Heavy Pages
Applied throttling reported LCP 6 to 33 % higher than Lighthouse's default simulation on three pages, and the simulated Speed Index ignored images entirely.
Lighthouse Audits 3 min read
How Much Do Lighthouse Results Vary Between Identical Runs?
On a local page, 15 Lighthouse runs all scored 100 with LCP within 3 ms. On our live home page, ten runs scored 94 to 97 while LCP spread from 1.5 to 2.6 seconds.
Lighthouse Audits 3 min read
Lighthouse ‘Explicit Width and Height’: Which Sizing Methods Pass
Lighthouse 13.5 accepted width and height attributes, CSS pixel sizes and CSS aspect-ratio, and flagged images with no size, a width attribute alone or a CSS width alone.
Lighthouse Audits 3 min read
Lighthouse Aspect Ratio Audit: It Flags Distortion You Cannot See
Lighthouse 13.5 flagged images stretched by as little as 0.9 %, while visible distortion started around 10 %. The fixes: size by one dimension, crop with object-fit.
Lighthouse Audits 3 min read
Lighthouse ‘Serves Images With Appropriate Resolution’: Where the Line Is
In a 300-pixel slot on Lighthouse's 1.75× mobile profile, files of 300 pixels or less were flagged as too low resolution and files from 350 pixels passed, against a stated expected size of 450.
Lighthouse Audits 3 min read