Website performance: images, JavaScript and Core Web Vitals
Diagnose slow loading, delayed interaction and layout shifts, then prioritise image delivery, JavaScript work and repeatable measurements.
ecomvision editorial team
2 min
Published
Key takeaways
- 01Distinguish loading, responsiveness and visual stability.
- 02Measure representative pages and real journeys.
- 03Fix the largest observed bottleneck first.
Identify the slow part
LCP measures the arrival of major content, INP interaction responsiveness and CLS unexpected layout shifts. Good-experience targets are LCP at most 2.5 seconds, INP at most 200 ms and CLS at most 0.1, assessed at the 75th percentile.
Lab tests help reproduce issues; field data reflects visitors. One fast desktop run does not represent every user.
Optimise major content
Provide dimensions, appropriate formats and responsive image sizes. A small card does not need a multi-thousand-pixel original.
Do not casually lazy-load the main above-the-fold image. Verify the actual LCP element instead of assuming it is always an image.
Review main-thread work
Large client libraries and heavy handlers can delay input after loading. Separate static content from interactions and inspect third-party scripts and offscreen work.
Compare effects enabled and disabled on an ordinary device.
Prevent unexpected movement
Reserve space for images and embeds. Review font changes, notices and content inserted above the reader.
Repeat the same measurement after a fix and check that image quality and accessibility remain intact. Record testing conditions, not only a score.