amdad performance is important. But for Flarum, extensibility is also key point. We recently tested the available options again before deciding to update to Mithril v2, and so far Mithril remains the most extensible of them based on our research. We're of course open for new framework suggestions, they might come in useful in a next big update.
I just had a look at the Svelte tutorial and examples, and I don't immediately see how components can expose extendable parts. It's almost a necessity for components to be javascript classes to allow monkey-patching in an extension-based system like Flarum.
I actually think having a virtual DOM is useful for us, because it allows extension a lot more freedom to inject new things in various places, without having to recompile all other extensions to account for it, as I suspect Svelte might require.
