Glossary
Page weight
Page weight is the total number of bytes a browser downloads to display a page: the HTML document plus every stylesheet, script, image, font and media file it references. It is the crudest performance measure available and remains useful because it is unambiguous. On most sites images dominate it by a wide margin, and JavaScript costs more than its share of the bytes because it must also be parsed and executed.
What typically dominates
Images, usually more than half the total on a content site, and the easiest to reduce without changing anything a visitor notices.
JavaScript, which is smaller in bytes and far more expensive per byte: it must be downloaded, parsed, compiled and executed, all on the main thread.
Web fonts, several files when multiple weights and styles are loaded, each typically 20 to 100 kilobytes.
Video, which can exceed everything else combined when it autoplays.
Third-party resources, which frequently account for a third or more of the total and are the part a site owner has least visibility into.
Compressed and uncompressed
Text resources are compressed in transit, so the transferred size is far smaller than the file on disk — typically a quarter for JavaScript and CSS.
Tools report both, and the numbers are answering different questions: transfer size is what the network carries, and decoded size is what the browser must process.
For JavaScript the decoded size is the one that predicts execution cost, which is why a 200-kilobyte transfer can be more expensive than a one-megabyte image.
Images and video are already compressed, so the two numbers are nearly the same for them.
What weight costs
Time on constrained connections. On a slow mobile network, each additional megabyte is measured in seconds.
Money, for visitors on metered data plans, which remains a substantial share of mobile traffic in many markets.
Battery and processing on lower-end devices, particularly for JavaScript.
It is a proxy rather than a metric: a heavy page that loads its visible content first can feel faster than a light page that blocks on a slow server. Weight and the loading metrics have to be read together.
How to measure and reduce it
The browser's network panel reports the total transferred and the breakdown by resource type, which immediately identifies what dominates.
Measure with the cache disabled, since a returning visitor's experience is not the one that decides whether someone stays.
The reductions in order of usual return: serve images at display size in modern formats, remove unused JavaScript and CSS, subset fonts, audit third-party scripts.
Set a budget and check it in the build. Page weight grows monotonically without one, because every individual addition is small.
Frequently asked questions
- What is a reasonable page weight?
- There is no defined limit. A useful discipline is a budget appropriate to the audience's connections, enforced in the build rather than reviewed occasionally.
- Why is JavaScript more expensive than images of the same size?
- Images are decoded off the main thread. JavaScript must be parsed, compiled and executed on it, so the cost continues after the download.
- Does page weight affect rankings?
- Not directly. It affects the loading metrics that feed page experience, and it affects whether visitors wait.
Sources
Related
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