You can't. Apple is no longer signing/authorising older versions of iOS for attempts at downgrading.
In, addition, Apple does NOT have any support for downgrading iDevices to a previous iOS version.
I have upgraded two iPad 2s multiple times to iOS 9.1 and they are functioning fine on iOS 9.
Try resetting All Settings found in the Settings App under General and in the right column under Reset.
Try resetting histories and deleting/resetting caches in all of the web browsers you use.
If you use Safari, these functions maybe l in the Settings App under Safari.
Other web browsers have their settings inside of the running app, itself.
Try turning on Reduced Motion.
This is found in the Settings App in the General tab the left panel.
In the right panel look under Accessibility, the look for Reduce Motion and turn this feature "On".
You should see an appreciable performance increase on all iPad 2, 3 and 4 models.
In the Settings App under the General Tab, in the right column, look for Background App Refresh and turn this setting to "off".
Try a reset of your iPad by holdings down both the Home and sleep/wake buttons simultaneously until your iPad goes to black and restarts with Apple logo, then release the buttons.
Good Luck!