Ten people load the same webpage. Nine of them see it in about 12 milliseconds, basically instant. The tenth person hits a slow server and waits 4 full seconds.
Now someone asks: "So, how fast is our page?"
You do what feels obvious. You average the ten numbers.
410.6 milliseconds.
Write that in a report and it looks fine, even respectable. But sit with it for a second. Whose experience does 410ms actually describe? Not the nine people who got 12ms. Not the one person who got 4000ms. It describes a person who doesn't exist. One outlier dragged a "typical" number to 30 to 40 times higher than what 90% of your users actually felt.
