Do you mean speed in development or performance? .NET 6 has made major improvements on both fronts.

The best answer I have is the recent Blazor presentation from Steve Sanderson (the original author) from NDC Oslo conference on 2/21/2022: https://www.youtube.com/watch?v=Rn8psTi8FBk

It's 1 hour and will explain in far more detail exactly how good it is, from hot-reload (which is massive), importing other packages (including native Rust or C source like SQLite), unified server/wasm APIs, prerendering/hydration improvements, EF Core wasm, and even integration with existing JS react/vue components.

The other mode of Blazor, server side, is where Blazor really shines. Performance is very fast on the client, and it's a much simpler experience for development that traditional SPA while having a very similar programming model. It's simpler partly because of the cohesive much simplified stack, and partly because for most simple one client apps you don't need to create a server API with all the extra work and issues that brings vs having your server side data in process.

Blazor WASM is still unfortunately slow on load, however the recent Steve Sanderson video on Blazor in .NET 6 showed impressive client runtime performance once loaded, by doing virtualized UI access to in process Sqlite for a truly native client performance much better than the current gen SPAs and getting back to how good software used to be, literally instant scrolling over tens of thousands of records.

But Blazor server is really excellent, recommend it.

It is still quite slow, especially at first load due to its comparatively massive payload size. Server-side depends entirely on the connection speed of the interfaces and hardware involved, and will always be necessarily verbose.

In typical MS tradition, there is a naming issue. Blazor can mean client-side (via WASM) or server-side.

I can see why this is named this way, considering both variants share a lot in common, aside from some platform specific considerations (parallelism, and file system, etc.) moving from one platform to other is fairly simple. In fact back when Blazor WASM didn't provide decent stack trace on a browser console, I was doing development on Blazor Server and then ported it over to WASM.

Still, it is not Angular level slow while being easier to use.

While your gut may be telling you that, no actual data bears that out as a reality. Exhibit A: a set of benchmarks with actual rationale and cited sources: https://www.diva-portal.org/smash/get/diva2:1578257/FULLTEXT...

From an objective standpoint it's not a valid comparison. Blazor still has a number of fundamental deficiencies in regards to how it interfaces with the DOM event model via its messaging abstraction. You can't create a symmetrical set of tests without dropping concerns Blazor can't accomplish without custom JS, or limiting the scope of comparisons to what Blazor can do.