Ipad fast charging

I would first ensure that my adapter is appropriate, with a PD Compatibility. You can read the answer in a previous thread here : https://discussions.apple.com/thread/252099783 for details about charge negociation.

From there, use Settings / Battery to monitor how fast your iPad is charging. Battery settings provide the charge history. The charge periods are indicated with a small Power (lightning) icon. Try charging for example from 20% to 100% using each adapter and compare how fast your device is charged.

Hope it helps.

New 2020 iPad Air 4 DOES, indeed, support the PD standard fast charge protocol.

The 2021 iPad Air IS entirely based upon the 2018, 2020 and 2021, 11 inch screen iPad Pro models, all of which, also, suport the PD standard fast charging protocol.

With your current Apple 18 or 20W power/charging adapter and USB-C cable that is included with your 2020 iPad Air 4, charging time should be anywhere between 2-1/2 to 3 hours.

Best of Luck to You!