The contributing factors.
WHAT AFFECTS HOW FAST A PAGE FEELS
How quickly the server responds How much must be downloaded How many requests are made How much work the browser must do Whether content appears progressively Whether the layout shifts
WHAT MATTERS MOST TO USERS
How quickly something useful appears, and how quickly they can interact.
Not when everything has finished.
WHAT TO MEASURE
Time until meaningful content is visible Time until the page responds to input Whether content moves after appearing
WHY THAT LAST ONE
Shifting layout causes people to click the wrong thing.
WHAT USUALLY DOMINATES
Images, scripts and fonts.
WHAT TO MEASURE ON
A modest device, on a slow connection.
WHY
Development machines and fast connections conceal every performance problem.
WHAT MATTERS LOCALLY
Data cost as much as time. Visitors pay for every byte.
WHAT TO ESTABLISH
Total transferred size, and what the largest items are.
WHAT TO DO FIRST
Address the largest item.
