Most sites are fast on launch day and slow six months later, because nobody re-checked the things that quietly degrade. The habits below are the ones we run on every project we operate.
Measure before you optimise
Set up real-user monitoring from day one — not just lab numbers, but field data from actual visitors. When a slowdown appears, you want to know which page, which region and which device class it hit.
Keep the cache honest
Caching fails silently: stale headers, cache-busting that busts everything, or a CDN rule that lets the most expensive page through uncached. Review cache hit-rates monthly and treat a falling hit-rate as an incident, not a trend.
Images are the first budget line
On most content sites, images are the largest payload after the HTML itself. Modern formats, responsive sizes and lazy-loading below the fold remove most of the problem without touching the design.
Load-test the growth, not the launch
Test at the traffic you expect in a year, not the traffic you have today. The database index that is fine at 100 queries a second is the one that breaks at 1,000 — and it is cheap to find out now.