bought the iPad mini 2 last year

Downgrading a device's iOS version isn't supported by Apple. What have you tried to see if it improves things e.g. have you checked Settings, done a soft-reset/reboot, backup up and restored, reset to factory defaults (after copying off anything that you want to keep) and just resynced content ?

Have you tried a hard reset of your iPad by holding down both the Home and sleep/wake buttons simultaneously until your iPad goes to black and restarts with Apple logo, then release the buttons?