iOS 11 on iPhone 5s: Should I expect a decrease in performance?
I own an iPhone 5S but I have not yet upgraded to iOS11. That is mainly because I like to let all the bugs get worked out first. Once it hits 11.1, I'll upgrade. However, to answer your question, take a look at this article that goes through a detailed evaluation of performance on the iPhone 5S. Overall, it doesn't seem to affect it to negatively.

Conclusion from the article:

Should you update?

I could understand wanting to hang around on iOS 10 if you have an iPhone 5S. Apple's oldest supported iPhone indisputably loses some of its pep in the move to iOS 11, and while it gets a lot of the same things as other newer devices, it does miss out on things like ARKit, the improved Siri voice, and some contextual Intelligence features.

But as I do every year, for most people I would come down in favor of updating. The iPhone 5S is slower with iOS 11, sure, but it's not as slow as the iPhone 4S was with iOS 8 or 9, or even as slow as the iPhone 5 is with iOS 10 most of the time. And as we mentioned, you still get a bunch of new iOS 11 features, and you're bound to find something worth upgrading for in that list.

Even more importantly, though, you need to be on iOS 11 to get new security updates at this point. Apple only very, very rarely issues any kind of patch for older iOS versions after the newest one is out, so the first disclosed vulnerabilities that come to light after today are only going to get patched in iOS 11. Really, practically, you can probably afford to wait around until iOS 11.1 barring any major catastrophes—those updates usually come out toward the end of October—but in the long run it's not safe or responsible to use old unpatched operating systems indefinitely.

So, update. If not today, then soon. The iPhone 5S gets a little slower, but that's how it goes when the oldest hardware that runs an operating system is only 20 or 25 percent as fast as the most recent hardware. It'll never be as fast as it was, but it's fast enough for a budget or hand-me-down phone, and it keeps doing new things; that's an acceptable trade-off.