My personal gripe with the eShop is the horrible startup time (compare to the time it takes the settings application to open - part of this can be assigned to it being a web application indeed), lack of partial/incremental presentation, and possibly lack of prefetching. Asset loading causing bad UX is always a frontend problem.
Sure, you can write slow code in any language, but the language will limit how fast your code can ever be.
This from a Python fanboi, FWIW. I'd never try and use Python on an embedded device where performance matters, not only for UI responsiveness, but for battery life.
Some things will never JIT. Some things will JIT much later, leading to sudden CPU spikes later. And importantly, code will often deoptimize a few times, meaning that it drops back to interpreted to re-trace even if it had already optimized this code block.
And as most web "applications" are loaded only to be thrown away as you finished reading fixed text, all of those resources were burnt for nothing. This includes an application like the eShop, which has limited need for JS for anything other than lightweight asset handling.
IME, it's not the presentation, is the loading of assets into memory. Once the assets (like thumbnails, descriptions) are loaded, the display is as fast as always, but you scroll beyond what's cached and it turns back into molasses.