Load speed and layout stability directly affect how many visitors leave a request - and how many leave for a competitor before the page even finishes rendering.
The most common mistake is optimizing only for first render speed while ignoring layout shifts caused by fonts and images loading in. Those are exactly what annoys mobile users the most.
A practical baseline: compressed modern image formats, fonts with a sane fallback, and explicit sizes for media blocks - that alone closes most of the gap without a deep rebuild.