Unoptimized native application, probably written in a yolo let’s ship it faster way, is slower than rewrite that learned from mistakes of previous application. What makes you surprised?

Though JS is single-threaded, it only builds the render tree and is more than fast enough if you’re careful. RN has a separate UI thread for actual rendering, I/O is offloaded and extensions can use extra threads.

The largest bottleneck historically was the bridge between obj-c and js (now superseded by JSI), and not the speed of the engine itself.