Blazor Web App WASM download performance issues

[wendellestradairely]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

There are too many wasm files being downloaded during startup and every time I reload the browser. It took almost 10 seconds to load all these files in local environment and several minutes to load these files into the browser when deployed to a remote/cloud server.

This happens only in .NET 10 rc1 but not in previous versions (e.g. 9.0/8.0).

Expected Behavior

Must load faster like in .net 8/9

Steps To Reproduce

Compare two simple projects in both .NET 10 and .NET 9/8

1. Create a new Blazor Web App.
2. Use InteractiveAuto
3. Global Interactivity location

Refresh/reload the browser. As you can see in .NET 10 it's download several wasm files and are very slow. This behavior doesn't happen in .NET 9 and .NET 8

Exceptions (if any)

No response

.NET Version

No response

Anything else?

No response

[javiercn]

@wendellestradairely thanks for contacting us.

During development asset caching is disabled to avoid serving stale assets. In production you should see the files served from cache. In .NET 8.0 and 9.0 we used a custom browser cache object to avoid the asset download altogether, but that came at a cost of size and complexity.

In .NET 10.0 an onwards we simply rely on the browser HTTP cache, which shouldn't show any difference in performance or behavior provided you are publishing in Release mode and running the published app.

During development we take some shortcuts to avoid potentially serving stale assets.

Please publish the app in Release mode, run the published output and let us know if you still have issues in that configuration.

[wendellestradairely]

@javiercn It's already published in Release mode. I wish I could share the app link, but I could only show a video recording.

Here's the comparison between the same apps published in both .NET 10 and .NET 9. In .NET 10, it took over a minute for the page to become interactive whereas in .NET 9, it only took about 5 seconds for the page to become interactive. The issue happens on every normal page refresh.

(P.S. Don't mind the timer in the recording it's not accurate. Please see the browser's Network tab on the right.)

.NET 10 recording (trimmed)

Recording.2025-10-08.135337.mp4

.NET 9 recording

20251014-1228-36.8339721.mp4

Same thing happens in Starter template (dotnet new blazor -int Auto -ai).
How I published the app (tried 3 methods):
dotnet publish
dotnet publish -c Release
Publish via Visual Studio Insiders (both in Azure App Service and On-prem IIS)

[javiercn]

@wendellestradairely what does the from column after time say?

[wendellestradairely]

@javiercn You mean the "Fulfilled by" column?
It says nothing. It's blank.

Only the css and javascript are either memory or disk cached

[wendellestradairely]

Clear site data + Hard refresh = wasms are being downloaded with status 200
Normal refresh = wasms are always being downloaded with status 304

[wendellestradairely]

Here's an example using the starter template. Same issue.

[wendellestradairely]

UPDATE:
I have upgraded to the latest .NET 10 RC.2 (10.0.0-rc.2.25502.107). I can no longer see a bunch of wasm files being downloaded on every browser refresh. It loads faster now. However, you can see a bunch of console errors.

Another issue is that it doesn't transition to WebAssembly render mode even after several reloads:

[oroztocil]

@wendellestradairely

The errors you see in the console are caused by an unrelated problem with Wasm publish that was introduced in RC2 and reported elsewhere (#63970, #64041). This problem is fixed in the latest build.

Could you please try to reproduce your original download issue with the latest RC2 build (currently the same as the latest main)? You can get it from here: https://github.com/dotnet/dotnet/blob/main/docs/builds-table.md