Aim for the main content visible inside ~2.5 seconds on a phone — that's the threshold Google's own Core Web Vitals ('Largest Contentful Paint') defines as good, and it's where visitor patience measurably runs out.
Speed compounds quietly: it's a ranking input, a Quality Score input for ads, and a conversion input all at once. The usual culprits are oversized images, tag bloat, and cheap hosting — boring fixes, big returns.
Test with Google's free PageSpeed Insights on your top pages (not just the homepage), on mobile, and fix the largest flagged item first rather than chasing a perfect score. Past 'good', speed work hits diminishing returns fast.