Is it worth upgrading to iOS 12 on an iPhone 5s
4 year old iPhone 5s owner. Upgraded to iOS 12. Experiencing noticeable speed improvements (in overall usage) with no noticeable impact on battery performance.

The overall experience is definitely better than running iOS 11.4.1 on the same device. However, note that the battery performance is highly dependent on your usage of the device. In my experience, the battery backup has remained the same compared to iOS 11.4.1.

iOS 12 on the iPhone 5S, iPhone 6 Plus, and iPad Mini 2: It’s actually faster!.

In-fact, all iOS device released in the year 2013 and later (iPhone 5s, iPod touch 6th generation, iPad Air, iPad mini 2) are eligible to run iOS 12, and, as per Apple, will see noticeable speed improvements due to under the hood performance improvements made in iOS 12.