I'm generally pro-C# and reducing developer effort however it rubs me the wrong way when simple UIs have lag.
About the payload size of the app, this is still very much a work in progress on all fronts, whether it being missing WebAssembly features, size optimizations on the dotnet front, or linking/trimming (IL and native) opportunities on Uno's generated code side.
On the performance side, threading is coming, AOT (IL to Wasm) is improving significantly in .NET 7 and more coming in .NET 8 to reduce the size of the payload by using newer WebAssembly features (albeit at the expense of older browsers).
As for the menus and toolbars, you can take a look at some our our live samples here: https://playground.platform.uno/#menubar
We know that Wasm-only apps will still be at a disadvantage for while when compared to JS frameworks simply because browsers are offering a lot out of the box (standard JS library, GC, Runtime, HTML layouting/rendering, more advanced JIT, etc...) but that gap may eventually narrow down as Wasm evolves, making the porting and development of webapp using many established language ecosystems an attractive choice.
I think the problem with XAML is, that it is not really declarative. Declarative would be: "I want a list/detail view, and the detail view has the following actions." and then you apply a theme for look&feel. Rather, you quite explicitly place rectangles, borders, and so on, in a scene. In UNO, a button doesn't translate to a HTML button, but to four nested <div>s and a <p>.
It even seems to generate proper HTML, instead of using a canvas like some other solutions, which sidesteps a plethora of issues.
That said, loading the gallery meant 24 MB of assets, 6 MB with compression still, which does feel kind of heavy. That said, I kind of understand why, seeing as dotnet.wasm was about 12 MB of that (a bit above 2 MB when compressed).
So in a sense, we're getting close to being the same size as React, Angular and Vue applications, though still an order of magnitude off, which may not be good enough for some.
That said, loading the front page of Reddit (new design) is also about 7 MB of content, so clearly people prioritize the content/looks over download sizes, though the overhead of WASM solutions still cannot be ignored.
On the desktop, however? It feels to me like more non-packaged-webapp options are nice, even like JavaFX/OpenJFX since the performance and resource usage of those can still be better than that of a packaged browser. One might also suggest to always try to use the native options, but I'll take what I can get.