I've used my iPhone as a hotspot for my laptop during a business conference - as well as for my boss. Phone didn't get hot, didn't see abnormal battery issues. But I always carry around this: https://www.amazon.com/Anker-PowerC...F8&qid=1524059882&sr=8-5&keywords=anker+10000 wherever I go in my backpack. So I can power my phone for events like this if needed. Also helps keep my phone powered during 8+ hour flights without a sweat.
Depends on what you're doing like C DM said. Taking notes in Word? Almost no usage. Watching Youtube? Probably a bit more.
Doing notes in word - almost no noticeable impact but I do have an 8+ and have had the plus model for the last 2 iPhones. (6+, 6s+, 8+).
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