[averne_]

Because it runs in a browser (WebKit-based), and because the browser is compiled without JIT for security concerns. It should really be a native applet, but Nintendo probably wanted to control it without needing a full firmware update.

[danuker]

They could have simplified the payload then (layout, JS...).

[averne_]

I'm not sure there's much to be simplified, interpreted JS is just that slow. In more recent firmwares Nintendo introduced more security-oriented changes (CFI, PAC) that potentially slowed the browser down even further. The Switch CPU is also not fast to begin with, and severely underclocked compared to what you would see on a regular Tegra X1. It used to be that you could access the Switch eshop using a dumped console certificate, and it was very smooth on a regular desktop browser.