Speed work gets reduced to "install this plugin", which is why so many sites stay slow after paying for optimisation.

The three that actually matter

1. Images, sized properly

This is the single largest cause of slow pages. An unoptimised 4MB photo served at full resolution is usually responsible for more delay than everything else combined.

Serve the right size for the screen, compress it, and let the browser pick. That one change regularly halves the page weight.

2. Not shipping JavaScript you do not use

Every kilobyte of script is parsed, executed and often blocks rendering. Most sites carry a library for a feature that could be twenty lines of code.

3. Fonts that do not block the page

Self-hosted fonts, loaded without blocking the first paint. A site that waits for a font before showing anything feels slow even when it is not.

What does not matter much

Meta tags. Invisible to the user.
Keyword density. A 1990s idea that never worked.
Most caching plugins. Helpful, not transformative.

Measure it properly

Test on a real phone, on mobile data, not on your office wifi. Most of your visitors are not on your wifi, and your laptop is not a phone.

The numbers that matter are Largest Contentful Paint and Cumulative Layout Shift — not a score out of 100 that rewards you for things nobody sees.