Alright, what the root cause?
The new iOS 8 works seamlessly on iPhone 4S, but the apps you already have on your iPhone are the reason why your battery drains faster and the performance is spoiled.
What's the solution to it?
After you upgrade to iOS 8 take a backup of your important data (Not Apps) and Restore your iPhone as a new Phone with Factory Reset. Once you are done with Restoring your iPhone you will notice that the battery and performance is just as it was with iOS 7.1.2.
What about the apps I want to use?
When you are ready with your restored iPhone—start installing the applications one-by-one to spot the culprit.
I am using iOS 8 on my iPhone 4S and I have gone through the issues. I worked on all available options like downgrading to iOS 7.1.2, backing up all the apps and restoring the apps—eventually, the findings above have helped me enjoy the new iOS 8 without any issue.
Upgrade to iOS 8 on your iPhone 4S and enjoy the new features folks!