Some comments and ideas:

Probably none of which are helpful.

1) I've had this to some degree on almost every update since Steve Jobs was safely buried. I didn't buy into the cult of Jobs until he died and Apple's quality deteriorated and innovation disappeared, so to a certain degree it is "normal"

2) Since the above, updates switch s**t back on that you turned off so check for settings that you don't want - bluetooth always gets turned back on for example. Check that the personal hotspot is off (that would be your mistake though), check the location services options and background update options if you don't need them (can do those app by app).

3) Which phone do you have? The older ones famously had a built in option which slowed down iOS as the battery got low to preserve the lifetime of the battery. Recent updates made that an option. If the option is turned on.

4) More about all battery tips in 11.3 here (scroll down a bit):

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Posted By: Steve in Holland, Apr 10, 14:08:01