iPad charging slowly after being uncharged for a long time

If you experience difficulties charging your iPad when using an appropriate Power Adapter, preferably rated at 20W or greater, you would be best advised to seek assistance directly from Apple Support - and request a remote diagnostic test of your iPad.

You can contact the Support Team using the Support link at top-right or bottom-left of this page. Alternatively you can initiate and manage your support cases from Apple devices using the excellent Apple Support App. If not already installed, the App can be downloaded from the App Store:

https://apps.apple.com/app/apple-support/id1130498044

You might also visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to hands-on test and assess your iPad, Power Adapter and cable(s):

Genius Bar Reservation and Apple Support Options - Apple

Give it a few changes and see if get better. If not you may be a new battery. Take it to a local Apple Store and have them check it out.