why my ipadmini charge so slow

Make sure that you are using the iPad charger that Apple provides with the device and plug it into a wall socket. Don't charge the iPad from your computer. It takes way too long to charge that way - if it works at a ll. Not all USB ports supply enough power to effectively charge the iPad.

Make sure that you are also using the Apple supplied cable - not a third party cable. There are third party cables that work just fine, but if you want to rule out charger and cable issues, use the ones that Apple provides in the box with the iPad.

The 96% reading could be mean that the battery gauge needs to be calibrated. You could be getting a false reading about how power is actually in the battery. Read this for more information about the battery.

https://www.apple.com/batteries/ipad.html

This is what you can do to try to ensure that you are getting a correct reading.

Use Your iPad Regularly

For proper reporting of the battery’s state of charge, be sure to go through at least one charge cycle per month (charging the battery to 100% and then completely running it down).

However, it should not take 12 hours to charge the battery. That is probably twice as long as it should be. It certainly would not be a bad idea to take the iPad to an Apple Store or an authorized repair center and have it looked at.