Ios 9.2 and iphone 6

Try to reset your device. This won't delete your data stored on this device.

• Press and hold the Sleep/Wake button
• Press and hold on the Home button
• Keep holding both buttons until the display turns off and back on with Apple logo on it.

Alternatively, you may go to Settings - General - Reset - Reset All Settings

If this does not help, restore your device to factory settings. Note that this will erase the data from your device.

Take a look this Apple Support article: Use iTunes to restore your iPhone, iPad, or iPod to factory settings - Apple Support.

When exactly it is slow? Playing games and surfing internet, Skype conversations, using 100 apps at the same time tax the processor and may fell like your iPhone is slow.

How much free space is on your device? If it's not very much, this can cause the exact problem you're describing.