It's fast enough to make running an app feel like running a native app – faster start time, less memory used, less power hungry, more responsive UI – compared to browser based cross-platform apps.

One can say it achieves maximum user interaction speed.

For most critical calculations, one can easily integrate C++/C libraries – at the source code level – which Swift can compile, link and call directly.

Or drop down to unsafe features of Swift and achieve speeds comparable to C++. But that's not really needed in most of real life use cases. Convenience is what really matters when it comes to Swift.