Speed Matters More
Google measures page speed differently on mobile. A site that loads in two seconds on desktop might take six on a phone with a spotty connection. Core Web Vitals focus heavily on mobile metrics because that's where most ranking decisions get made.
Images often cause the biggest problems. A full-resolution photo that looks fine on a monitor becomes a massive bandwidth drain on mobile data. Compression, lazy loading, and proper format selection make measurable differences in bounce rates.