iOS 11.3 beta 5 has , identical performance compared to iOS 11.2.6, and here we are talking about a regression, because beta 4 has better performance. Specifically, iOS 11.3 beta 5 is faster in opening applications and running certain features, while iOS 11.2.6 is faster in others, so overall both versions are about equal.
iOS 11.3 Beta 5 How good is the iPhone and iPad
This is very easy to see on the iPhone 5S and iPhone 6, which are Apple's oldest phone models, but iOS 11.3 beta 5. Of course, the more we we are closer to iPhone 8, the performance of iOS 11.3 beta 5 is all the better, but you also need a new phone to take advantage of the good performance.
iOS 11.3 had much better performance in the first version released by Apple company, but here in iOS 11.3 beta 5 they have, unfortunately, made a small regression. Of course, iOS 11.3 beta 5 is, as the name implies, a beta version of the operating system, so up until the release of the final version should improve performance.