I use this 5S as my secondary phone, so I don’t really use it but would it be usable with the latest iOS?
In fact, not the worst but not the best, too. :D
After 4-5 hours of downloading iOS 12 on my 5S and SE, I could compare the two. I can't say anything about the battery life because in my 5S, my battery's health is 60%, in my SE, it's 90%, so not the best.
Here is is the comparison: