webp avif comes up in nearly every audit we run. Rarely because the topic is complex: more often because it is handled last, once the budget is already spent.
The mistakes we see most often
Three mistakes recur with surprising regularity: applying a recommendation without checking it fits the context, changing several parameters at once which makes measurement impossible, and giving up after six weeks because nothing moved.
The third is the most expensive. Effects are rarely visible before a full recrawl and re-evaluation cycle, which often takes two to three months on a mid-sized site.
Visual stability, the detail that irritates
Content jumping during load causes frustration out of all proportion to its technical cause, often trivial: an undeclared image dimension, a badly calibrated fallback font.
These are the cheapest fixes in all of web performance, and among the most perceptible to users.
A budget, or nothing
Without an explicit ceiling, a page grows with every change. Setting a budget — in weight, requests and render time — turns performance into a design constraint rather than an occasional project.
The budget must be checked automatically, otherwise it becomes a polite intention nobody consults.
Check before moving on
- Handle what affects the most pages first
- Measure before, not after
- Schedule a review in six months
- Remove before adding
Weight is not the only culprit
Cutting kilobytes helps, but load order often matters more. A lightweight render-blocking resource costs more than a heavy lazily-loaded image.
Start by identifying what blocks, then what weighs — in that order. The reverse leads to lengthy optimisation of elements with no perceptible effect.
Measure in the field, not the lab
A score obtained on a fibre-connected development machine says nothing about real experience. Field data, collected from actual visits, often reveals a considerable gap.
This is especially true on mobile, where network and CPU variability make averages misleading: look at the 75th percentile, not the mean.
Read next
Where to start
If you take only one action from this article, take the one touching the largest number of pages at once: the effort-to-impact ratio is almost always best there. The rest follows more easily.
