Add note on SqlClient async issues (#27747)

* Add note on SqlClient async issues

See dotnet/EntityFramework.Docs#4109

* Update aspnetcore/data/ef-rp/intro.md

Co-authored-by: Rick Anderson <[email protected]>

Author: roji

aspnetcore/data/ef-rp/intro.md

@@ -518,6 +518,9 @@ Some things to be aware of when writing asynchronous code that uses EF Core:
 
 For more information about asynchronous programming in .NET, see [Async Overview](/dotnet/standard/async) and [Asynchronous programming with async and await](/dotnet/csharp/programming-guide/concepts/async/).
 
+> [!WARNING]
+> The async implementation of [Microsoft.Data.SqlClient](https://github.com/dotnet/SqlClient) has some known issues ([#593](https://github.com/dotnet/SqlClient/issues/593), [#601](https://github.com/dotnet/SqlClient/issues/601), and others). If you're seeing unexpected performance problems, try using sync command execution instead, especially when dealing with large text or binary values.
+
 <!-- Review: See https://github.com/dotnet/AspNetCore.Docs/issues/14528 -->
 ## Performance considerations