If you check the compatibility of iOS9, you can see that iPhone 4S and iPad2 are supported by iOS9. But all iOS9 features will not be available for older version of iPhone and iPad. The users of older version of iPhone/iPad have reported slowness after updating to iOS9. Here are some solutions you can try to fix this issue:

Kill reduce motion

Go to Settings > General > Accessibility > Reduce Motion, and turn it off.

Perform a Hard Reboot

Please make sure you have kept a backup of your device data. To hard reboot your phone, hold down the Power button and Home button together at the same time until the Apple logo appears.

Wipe and Restore the iPhone or iPad

After backing up your device to iCloud or iTunes, reset the device to factory settings.

Clean up Your Old iPhone/iPad Completely

You can remove the various kinds of junk files like app cache, cookies, temp files on your device to make old iPhone iPad run faster. You can also use some iOS cleaning tool for this purpose.