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'm generally pro-C# and reducing developer effort however it rubs me the wrong way when simple UIs have lag.