Why apple 12w charger charging faster than Apple 20w charger for iPhone

Hello,

Hmm, you might check the charging port for lint or other debris:

Fast charge your iPhone - Apple Support

Also see this:

When to charge your iPhone or iPad

Good luck.

The 20W charger is rated for Power Delivery and will charge faster with devices that support that mode of fast charging (your iPhone X does not). One of the Apple magazines did a test back when the iPhone X was new and found that the 12 watt charger was the sweet spot for the iPhone X for best charging. And this test was not just with Apple chargers. To get a real comparison take the phone down to 20% and then charge it to 80% and see what the time is for both of them.

me too i used it on my iphone 7 plus, its supposed to be at the same speed but it is slower than the normal one