I almost always turn off cellular data when I lock the screen. I only turn it on when I’m using internet. But it’s annoying to do that almost every time. Does it worth the trouble?
You can follow general rules to optimize your battery life.
Now respecting your question, you save a lot of battery by turning cellular data only when you needed. Cellular data consumes a lot of battery power, specially when yo have have no coverage or low signal:
"No Cell Coverage and Low Signal. This indicates that either you are in a poor cell coverage area and your iOS device is searching for a better signal or that you’ve used your device in low-signal conditions, which has affected your battery life." This is according to previous apple link, however, take into consideration that GPS works with cellular data so if you leave it by accident powered on, your battery charge will decrease dramatically.
Also I recommend you to have your iPhone up to date, for possible bugs, of course back-up first.
Pay attention on one detail, if you back-up an iPhone with already performance issues, the iOS or iCloud back-ups could be affected too. So it is important to back-up often and not wait to have problems to back-up before an up date or even a restore.
My subjective opinion is that while yes, turning off cellular data would extend the battery standby time, you would see more significant improvement by simply enabling low-power mode on the battery.
This is dependent on your individual usage patterns of course.
The material of the above seems to indicate that the simple act of disabling the cellular-data in and of itself isn't saving power, should you not actually be using the cellular data to receive or transmit data. The fact that transmitting and receiving cellular data does consume battery power is what would lead to an increase in standby time ie. comparing the battery usage between a device with mobile data enabled (but not actively being used) and the battery usage of a device that has mobile data disabled, you would not expect to see a significant difference. Comparing the battery usage between a device with mobile data enabled (with high levels of push-service data being used) and the battery usage of a device that has mobile data disabled, you would expect to see a significant difference in standby time - hence the dependence on your individual usage patterns in this case.
Low-power mode disables various services and features, including mail fetch, Hey Siri, background App refresh, automatic downloads and some visual effects are reduced or turned off. I would expect the mobile data usage to be similar in both the cellular-data disabled, and battery low-power modes, with the low-power mode additionally disabling various processor intensive features.