I have found in my benchmark tests, that the performance of StringBuilder in .NET Core 2.2 and 3 is less performant than in .NET 4.8.
If string.Concat() uses StringBuilder, that is less performant too.
Results below:

Benchmark project located here: https://github.com/RealDotNetDave/dotNetTips.Benchmark.App
I have found in my benchmark tests, that the performance of StringBuilder in .NET Core 2.2 and 3 is less performant than in .NET 4.8.
If string.Concat() uses StringBuilder, that is less performant too.
Results below:

Benchmark project located here: https://github.com/RealDotNetDave/dotNetTips.Benchmark.App