Investigate Blazor WebAssembly startup performance characteristics

[MackinnonBuck]

Even when Blazor WebAssembly assets are cached, the time it takes for a Blazor WebAssembly app to initialize can be significant. We should investigate ways to further optimize Blazor WebAssembly initialization to reduce this startup time.

Some preliminary investigation suggests that Blazor-specific startup (everything that happens after the code in Program.cs starts executing) accounts for at least half the total initialization time.

We should:

• Add instrumentation to measure how long various initialization stages take
  • Such stages include loading cached assets, initializing the .NET WebAssembly runtime, and many of the steps between the start of the app's entrypoint and the first interactive render
• Collect traces for Blazor-specific initialization code
• Given those results, determine what work can be done to minimize WebAssembly startup time
• Consider benchmarking daily builds so we can track these detailed performance measurements over time

Related issues:

• Improve the load time performance of Blazor WebAssembly apps on low-end mobile devices #42284
• [Blazor] Auto Render not working properly in .NET 8 and page locks until WebAssembly start #52154

[zubairkhakwani]

Currently, when using Blazor in InteractiveAuto render mode, the application starts with Blazor Server on the initial load and transitions to WebAssembly once all DLLs are loaded and cached. However, on subsequent requests, it waits for WebAssembly to start, neglecting the benefits of Blazor Server.

To enhance performance and user experience, I suggest implementing a mechanism where the first page load or any subsequent refreshes always utilize Blazor Server, while booting WebAssembly in the background. This approach can mitigate concerns about WebAssembly startup performance, as we can improve it up to a certain extent.

Making this behavior configurable would provide flexibility. Users could choose to wait for WebAssembly to load when the cache is available or opt for Blazor Server on every initial request, even if WebAssembly is downloaded.

By implementing this approach, we can optimize the application's performance and provide users with a smoother transition to WebAssembly while still leveraging the benefits of Blazor Server where applicable.

Would love to hear thoughts and feedback on this proposal!

[mkArtakMSFT]

Bringing #31877 into the current milestone as @MackinnonBuck 's investigation leads us to believe that this work will bring substantial benefits to startup perf.