Page speed and website sustainability often improve together, but they measure different things. Here is what you need to know.
Clients have always asked us how they can improve their website speed and performance because of the SEO, user experience and conversion benefits that can come with a faster site.
Usually, someone has run a Google PageSpeed Insights or Lighthouse report and received a performance score out of 100.
More recently, a different request has started to appear. A client runs a sustainability scan using a free online tool and comes to us with a grade such as C or F.
So, are page speed and website sustainability the same thing?
Not quite. They overlap, but they measure different aspects of your website.
Speed Page speed is about how the website feels
Page speed is about how quickly a page feels ready to the person using it.
Google measures this through its Core Web Vitals, which look at how quickly the main content appears, how responsive the page is when someone interacts with it and whether content moves unexpectedly while the page loads.
This is why page speed is often discussed in relation to SEO. Google uses page experience signals as part of its ranking systems, while a faster, more usable website can make it easier for visitors to browse, engage and enquire.
However, speed tools are primarily measuring the user’s experience. They do not simply measure how much data has been downloaded to create that experience.
Sustainability calculators focus on data and energy
Website sustainability calculators estimate the environmental impact of loading a web page.
Tools such as Website Carbon consider factors including the amount of data transferred when a page loads and the carbon intensity of the electricity used to deliver it. If the website is hosted in a data centre recognised as using green energy, the estimate may be adjusted.
Page weight and hosting therefore have a significant influence on the result, although a calculator can only provide an estimate. It is not a complete assessment of an organisation’s digital environmental impact.
The Sustainable Web Design Model’s rating scale makes these estimates easier to understand by comparing page transfer sizes using grades from A+ to F.
An A+ represents the lightest 5% of pages in the underlying dataset. At the other end of the scale, pages transferring approximately 2.42 MB or more receive an F.
That does not automatically mean a website with a lower grade is badly designed. The grading thresholds were intentionally set at a challenging level to encourage the industry to build lighter, more efficient websites.
Where page speed and sustainability overlap
Thankfully, there is plenty of overlap.
Many of the improvements that make a page faster can also reduce its weight and estimated carbon emissions:
- Properly sizing and compressing images, including using modern formats such as WebP
- Removing JavaScript and CSS that are no longer being used
- Reducing third-party scripts, chat widgets, trackers and tag manager additions
- Lazy loading images and other content further down the page
- Limiting the number of custom font files
- Applying effective browser caching
- Reviewing whether large animations and videos genuinely add value
If a client asks us to improve one area, they will often see benefits in the other too.
Where they differ
A fast page is not necessarily a light page, and a light page is not necessarily fast.
Speed tools care about what the user sees and experiences first. For example, a hero video can sit behind a lightweight poster image. The page may appear quickly and receive a good performance score while several megabytes of video continue downloading in the background.
That may be good for perceived page speed, but not so good for sustainability.
Some performance techniques can also increase data transfer. Prefetching a page that someone might visit can make the next navigation feel almost instant. However, if the person never selects that page, the prefetched data has been downloaded unnecessarily.
It works the other way too. Moving to verified green hosting may improve a website’s sustainability result without making it noticeably faster. Equally, a slow server can make a lightweight page feel sluggish.
This is why neither score should be viewed in isolation.
So, what do you do with a low sustainability score?
Do not chase the letter grade at the expense of the website’s purpose or its users.
When a client brings us a low sustainability score, this is broadly how we approach it:
Check which page was scanned. It is often the homepage, which may be one of the heaviest pages because it contains more imagery, video and marketing functionality.
Identify what is creating the page weight. Images, video, custom fonts and third-party scripts are common causes.
Check the hosting. Confirm whether the website is hosted in a data centre recognised as using green energy.
Remove before optimising. Old carousels, unused tracking scripts and videos that nobody watches do not need compressing. They may need removing.
Optimise what remains. Resize and compress images, lazy load appropriate content and reduce unnecessary code.
Review third-party tools carefully. Chat platforms, analytics tools, advertising pixels and embedded content can steadily add weight and affect privacy, performance and maintenance as well as sustainability.
- Support the people managing the content. A significant amount of page weight can come from day-to-day uploads rather than the underlying code. Clear guidance on image dimensions, formats and video use can prevent the same problems from returning.
The right approach is to consider the website’s purpose, audience and business requirements, then make improvements that reduce waste without undermining usability or effectiveness.
Load what you need
Page speed and website sustainability are not the same thing, but they are closely related.
Page speed is concerned with how quickly and smoothly a website feels to the user. Sustainability calculators look more closely at the data and energy associated with delivering the page.
Build pages that only load what they genuinely need and you will usually improve both.
The tools, methodologies and grades will continue to evolve, but the principle is unlikely to change: lighter, more efficient websites can provide a better experience for users while reducing unnecessary digital waste.
Could your website be faster and lighter?
Spinbox can review your website’s performance, page weight, content and third-party tools to identify practical improvements that support user experience, SEO and digital sustainability.
Talk to Spinbox about website performance optimisation.