This is actually pretty common; it seems like each new version of iOS brings complaints about faster battery drain. Some say it's because you're using the phone more, exploring all the new features and updates. I've also heard the argument that the new OS is doing a bunch of rewriting or caching or something, and that battery life will even out after the first couple days.
With iOS 11, however, there may be another culprit.
If you've ever searched for tips on prolonging iPhone battery life, you've almost certainly encountered this one: turn off email fetch. The reasoning: battery life suffers if your phone automatically checks for new mail every 15 or 30 minutes.
That's why I always kept my accounts set to "manual," as I prefer to retrieve mail when I want to retrieve mail.
However, after installing iOS 11, I noticed there were frequently new messages waiting for me each time I opened the Mail app. So I checked my email account settings (which, incidentally, are no longer found in Settings > Mail -- sigh) and, sure enough, all my accounts had been set to Fetch!
What. The. Heck, Apple?
Needless to say, as described in Matt Elliott's, you're better off disabling Fetch. Here's how: tap Settings > Accounts & Passwords > Fetch New Data and change it to Manually.
Have you noticed a decrease in battery life post-iOS 11? If so, have you checked your mail accounts? I'm curious to see if others have encountered this unexpected (and most likely unwanted) change in mail-retrieval settings.