The team over at iAppleBytes put the iPhone 6s, iPhone 6, iPhone 5s, and the iPhone 5 through its pace running iOS 9.3.2 and iOS 10 beta 3. The videos show that iOS 10 beta 3 is as smooth and fast as iOS 9.3.2 in opening apps, with there being virtually no difference in speed between the devices running iOS 9.3.2 and iOS 10 beta 3.
With the final release of the iOS 10 a couple of months away, it is commendable to see Apple improve the system performance of the OS even on older devices like the iPhone 5s. However, since this is just a beta release of the OS, you should not read too much into the performance improvement as things can change considerably by the time the final version of the OS is released.