When does the new update come

I have 2 suggestions

a) you have something running on your iPhone that is draining your iPhone's CPU. Settings -> Battery and look at the Battery Usage.

If you tap on the clock icon to the right of "Last 7 Days" the Usage will display how much time was used as a background app, if the app runs in the background.

b) your battery is aging out, and iOS is throttling your CPU to avoid having your iPhone suddenly shutdown from lack of power, which could be life threatening should you need to make an emergency phone call when the battery runs out of juice.

Make a Genius Bar appointment at your local Apple store and have them check your battery.

There is a newer release then iOS 11.1 which is what is shown in your profile.

Try connecting it to your computer and then doing an update using iTunes on your computer.