Here are half a dozen things that you can do to get an optimized performance from your device.
Do you have an App that is hogging all the resources? 80/20 rule. If you see a few apps that are responsible for most of the issues, you may want to rethink about using these apps.
Settings -> General -> Battery Usage Settings -> Privacy -> Diagnostics & Usage ->Diagnostic & Usage Data
Applications often refresh in the background. Sometimes these can be responsible for your battery drainage issues.
Also turn off the location services.
Settings > General > Background App refresh
Settings > privacy > Location services
Disable push notifications for Apps that are of no interest to you.
Low Power Mode is a new iOS 9 feature that was created to preserve your. This can significantly extend your battery, but it also slightly lowers performance.
To turn Low Power Mode on go to Settings > Battery and you’ll see it at the top.
Please pay close attention to resource intensive games that you are playing on the device with full brightness. This will definitely lead to poor power management
Some people have noted that the Data usage is reportedly high after ios9 upgrade. One way to address this is to turn off the wi-fi assist feature in ios 9.
In the past, we have seen battery drain issues reported after major iOS upgrades. Please check the following articles for other tips.