Performance & Perceived Speed (a UX concern)

Speed is not an engineering metric that happens after design — it is a design material. Every layout decision, image choice, font load, and interaction reserves or spends a latency budget the user feels. Treat performance like contrast or hierarchy: a property you design for, measure, and defend.

Why performance IS UX

Latency is the most consistent driver of conversion, trust, and retention that exists across every product category.

• Conversion. Walmart, Amazon, and Akamai studies converge: every additional 100ms of latency measurably depresses conversion; pages crossing ~3s see bounce rates climb sharply (Google: bounce probability rises ~32% as load goes 1s→3s, ~90% by 5s).
• Trust. Users equate speed with competence and security. A janky, shifting, slow UI reads as "unfinished" or "unsafe" regardless of actual quality.
• Retention. Slow first sessions correlate with non-return. The first 5 seconds set the perceived quality ceiling for the whole relationship.
• SEO. Core Web Vitals are a Google ranking signal. Field (not lab) data, at the 75th percentile, decides pass/fail.

Human perception thresholds (design to these, not to round numbers)