As always when downgrading, time is of the essence as Apple could close the signing window at any time, making going back to a iOS 8.1.1 impossible.
I just want to tell you that iOS 8.1.2 is jailbreakable and if you feel that your iPhone is running slowly on iOS 8.1.2, you can downgrade right now..
How To Downgrade to iOS 8.1.1
STEP 1: Download iOS 8.1.1 for iPhone/iPad or iPod Touch.
STEP 2: Plug your device to your computer via the USB cable. Hold the Alt/Option key (Shift key if you’re on Windows) while clicking on the Restore option in iTunes.
STEP 3: A pop window will appear allowing you to choose the firmware you want to restore to. Choose the iOS 8.1.1 firmware you downloaded in step 1.
STEP 4: iTunes will now start downgrading your iPhone back to iOS 8.1.1. assuming that iOS 8.1 is still being signed by Apple. If it’s not, you will receive an error message and the downgrade will halt.