Performance regression for .NET 9.0 / .NET 10.0 vs. .NET 8.0

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@AndyAyersMS is already working on this.

Since Sep 2, 2025.

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Description

area-CodeGen-coreclr Priority:2 tenet-performance
Description

During benchmarking, we found that there is a significant difference between the performance of our project (RavenDB) between .NET 8.0 and .NET 9.0 or .NET 10.0.

Our results:

v7.1 (.net 8.0)
[OVERALL], RunTime(ms), 50225
[OVERALL], Throughput(ops/sec), 19910.40318566451

v7.1 (.net 9.0)
[OVERALL], RunTime(ms), 99615
[OVERALL], Throughput(ops/sec), 10038.648797871807

That is close to 50%(!) regression in our tests.

We were able to reproduce a similar regression in a much smaller benchmark, where we are observing roughly 10% performance regression between 8.0 and 9.0 (10.0 is a bit better than 9.0, it is still slower than 8.0).

Repro code here: https://gist.github.com/ayende/e94d9ad6e8bba41a36d33ffb9e6de2ad

Configuration
  • All tests were run on Windows
  • dotnet versions tests:
> dotnet --list-sdks
8.0.413 [C:\Program Files\dotnet\sdk]
9.0.304 [C:\Program Files\dotnet\sdk]
10.0.100-preview.6.25358.103 [C:\Program Files\dotnet\sdk]
Regression?

Significant performance regression

  • .NET 8.0 - 955ms - 995ms tested
  • .NET 9.0 - 1103ms - 1132ms tested

Full results:

PS C:\Work\tryout>  .\bin\Release\net8.0\ConsoleApp1.exe
955
PS C:\Work\tryout>  .\bin\Release\net8.0\ConsoleApp1.exe
989
PS C:\Work\tryout>  .\bin\Release\net8.0\ConsoleApp1.exe
995
PS C:\Work\tryout>  .\bin\Release\net9.0\ConsoleApp1.exe
1103
PS C:\Work\tryout>  .\bin\Release\net9.0\ConsoleApp1.exe
1119
PS C:\Work\tryout>  .\bin\Release\net9.0\ConsoleApp1.exe
1132
Data

Here is the profiling output comparing those two versions.
Note: Sparrow.Json.JsonOperationContext.ReadObjectInternal(Object, String, UsageMode, IBlittableDocumentModifier)

  • .NET 8.0 is 2,201 ms under profiler
  • .NET 9.0 is 2,847 ms under profiler
Image
Repro code

https://gist.github.com/ayende/e94d9ad6e8bba41a36d33ffb9e6de2ad

I run the build using dotnet build -c Release and change the <TargetFramework>net9.0</TargetFramework> in the .csproj from net8.0 to net9.0 to test the different frameworks.

@redknightlois @EgorBo

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