FileStream The System.IO.FileStream type has been rewritten for .NET 6 to provide better performance and reliability on Windows. Now, FileStream never blocks when created for asynchronous I/O on Windows. Profile-guided optimization Profile-guided optimization (PGO) is where the JIT compiler generates optimized code in terms of the types and code paths that are most frequently used. .NET 6 introduces dynamic PGO. Dynamic PGO works hand-in-hand with tiered compilation to further optimize code based on additional instrumentation that's put in place during tier 0. Dynamic PGO is disabled by default, but you can enable it with the DOTNET_TieredPGO environment variable. Crossgen2 .NET 6 introduces Crossgen2, the successor to Crossgen, which has been removed. Crossgen and Crossgen2 are tools that provide ahead-of-time (AOT) compilation to improve the startup time of an app. Crossgen2 is written in C# instead of C++, and can perform analysis and optimization that weren't possible with