New major updates to iOS are always welcome, but usually with new updates, there are new features that might require more work than before, and as such, older hardware might not be optimized to handle it so well. As a result, sometimes it can feel like installing a new iOS update on an older iPhone could result in downgraded performance.
For those who might be worried that this could happen with iOS 14 and are holding off on updating it, it seems that you might not need to worry. This is because according to a speed test and benchmarks conducted by iAppleBytes (see video above), it seems that an iPhone 6s running iOS 14 is holding its own just fine.
In fact, based purely on the video, it would appear that iOS 14 actually improves the experience on the iPhone 6s, where it seems that apps are less laggy and actually open up faster on the iPhone 6s with iOS 14 installed. The benchmarks are also rather telling where in terms of single-core score, it managed to maintain around the 2,500 mark (save for iOS 10 which fared the worst), and also around the 4,500 mark for multi-core scores.
Now, this isn’t to say that your experience will be the same. There are other factors that could hamper a phone’s performance, but assuming that there’s nothing wrong with your phone, then maybe consider updating to iOS 14 if you haven’t done so already.