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The iPhone 6 is a bit long in the tooth so wouldn't be surprised if your battery was on its last legs. That said, if it's only started with iOS 11, it will usually take a day or two for the iOS to settle and the battery impact to minimize. If it was my phone, I'd try:
1. Reset all settings (not erase) in Settings > General > Reset, and if that doesn't work, then
2. Erase all contents and settings
First one just resets all settings to stock, and you'll have to answer questions about notifications, location, etc. If that doesn't work after a few days I'd erase and start from a backup. If that doesn't work, erase and start as new.