WHY THE APPLE IPAD CHARGES MUCH SLOWER THAN OTHER APPLE DEVICES
Apple products don't necessarily have the best reputation of charge capacity, but to Apple's credit, they get away from it by having a very efficient operating system to drain as little power as possible when using multiple apps simultaneously which is a good compromise. However, that doesn't necessarily explain why the charge time on the iPad specifically is so long.
The reasons for this is largely due to the iPad containing a far bigger battery than both the iPhone range as well as a bigger battery than the Macbook range as well with a huge 7,606 mAh battery unlike the current iPhone 12 at 2,775 mAh and the Macbook Air M1 with 4,379 mAh.
With that said, the current iPad still uses a Lightning connector cable and not USB C which also drastically increases the charge time as well. Even with the high powered wall adaptor the iPad still takes over 2 hours and 30 minutes for it to fully charge up at 20W.
Take a look at the video below by the YouTube channel: Apple Explained on Why The iPad Charges So Slow...