Blazor webassembly startup speed and UI locking issue #65559

Description

When starting up mono and blazor the UI, e.g. input fields, are locked until the webassembly has finished processing.

This is not related to network speed and cannot be fixed by lazy loading as the mono runtime and subsequent blazor application still locks the UI while loading.

This is a larger issue because when prerendering first and then triggering the startup process the user experience is an instantly loaded page, that then hangs and locks all UI until page load is finished.
In the case where you succeeded in entering data before the UI locking startet, this too is deleted unless a mechanism to transfer input data from the prerendered site to the live app is implemented using javascript.

Reproduce:

1. Create a blazor webassembly app on .NET 6.0 LTS, with no authentication, configured for https and hosted on ASP.NET Core but without progressive web application features enabled.
2. In index.html add <input type="text" class="form-control" /> below <div id="app">Loading...</div>
3. In Index.razor add the following to see in console log when first render is processed to indicate the app has finished starting.
4. Publish the app in release mode to folder and run the generated exe file.
5. Navigate to https://localhost:5001
6. Check that the input field present while loading is locked, this is before the app displays on the white page with the "Loading..." text in the top right corner.

Configuration

Microsoft Windows 11 Pro
10.0.22000 Build 22000
AMD Ryzen 9 3900X 12-Core Processor, 3793 Mhz, 12 Core(s), 24 Logical Processor(s)
32.0 GB RAM
Firefox and Chrome browsers

&

iOS 12.2
iPhone 5s
Safari 12

&

iOS 15.4
iPhone 12 Pro
Safari 15

Data

Windows 11 PC:
Firefox console output:

Chrome lighthouse and performance profile results:
blazor performance chrome lighthouse and performance profile.zip

Firefox performance profile results:
Firefox 2022-02-18 10.41 profile.json.gz

iPhone 12 Pro:
https://github.com/proxy/user-images.githubusercontent.com/3007300/154679804-f67aa2d9-5c69-4f6e-9165-f5ed0681db5e.mp4

iPhone 5s:
https://github.com/proxy/user-images.githubusercontent.com/3007300/154679884-3bad3145-0e85-4a98-96ce-b890380f0b8c.MP4
https://github.com/proxy/user-images.githubusercontent.com/3007300/154679892-bb7bcd5f-7f78-4630-a334-867b216455bf.MP4

Analysis

As far as I understand part of the issue is that the mono runtime and systems that build on top are locked to using a single thread for everything including the UI, this causes the UI to freeze during initialization of the runtime and application.

If this was solved however a new problem would be users interacting with the prerendered site and then losing what they already filled out in input fields and so on as soon as the app loads unless the developers implemented a function to handle this case on a per site/input field basis which does not seem like a scalable or elegant solution.

I suggest mono and blazor allow for use of multithreading in wasm to allow interaction during load and suggest an automatic mechanism for transferring input in prerendered fields to live fields once the app is loaded.

For reference to other issues tracking a similar topic i refer to:
dotnet/aspnetcore#17730
mono/mono#12453
WebAssembly/threads#163