Add note on SqlClient async issues in perf docs (#4161)

Closes #4109

entity-framework/core/miscellaneous/async.md

@@ -19,7 +19,7 @@ For more information, see [the general C# asynchronous programming docs](/dotnet
 > EF Core doesn't support multiple parallel operations being run on the same context instance. You should always wait for an operation to complete before beginning the next operation. This is typically done by using the `await` keyword on each async operation.
 
 > [!WARNING]
-> The async implementation of [Microsoft.Data.SqlClient](https://github.com/dotnet/SqlClient) unfortunately has some known issues (e.g. [#593](https://github.com/dotnet/SqlClient/issues/593), [#601](https://github.com/dotnet/SqlClient/issues/601), and others).
+> The async implementation of [Microsoft.Data.SqlClient](https://github.com/dotnet/SqlClient) unfortunately has some known issues (e.g. [#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.
 
 > [!NOTE]
 > EF Core passes cancellation tokens down to the underlying database provider in use (e.g. Microsoft.Data.SqlClient). These tokens may or may not be honored - consult your database provider's documentation.

entity-framework/core/performance/efficient-querying.md

@@ -209,6 +209,9 @@ For more information, see the page on [async programming](xref:core/miscellaneou
 > [!WARNING]
 > Avoid mixing synchronous and asynchronous code in the same application - it's very easy to inadvertently trigger subtle thread-pool starvation issues.
 
+> [!WARNING]
+> The async implementation of [Microsoft.Data.SqlClient](https://github.com/dotnet/SqlClient) unfortunately has some known issues (e.g. [#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.
+
 ## Additional resources
 
 * See the [advanced performance topics page](xref:core/performance/advanced-performance-topics) for additional topics related to efficient querying.

entity-framework/core/providers/sql-server/index.md

@@ -33,6 +33,9 @@ Install-Package Microsoft.EntityFrameworkCore.SqlServer
 >[!TIP]
 > The Microsoft.Data.SqlClient package ships more frequently than the EF Core provider. If you would like to take advantage of new features and bug fixes, you can add a direct package reference to the latest version of Microsoft.Data.SqlClient.
 
+> [!WARNING]
+> The async implementation of [Microsoft.Data.SqlClient](https://github.com/dotnet/SqlClient) unfortunately has some known issues (e.g. [#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.
+
 ## Supported Database Engines
 
 * Microsoft SQL Server (2012 onwards)