Fix performance degradation in converters because the instance was not reused.

Author: cincuranet

src/EFCore/Storage/Json/JsonConvertedValueReaderWriter.cs

@@ -16,6 +16,9 @@ public class JsonConvertedValueReaderWriter<TModel, TProvider> :
     JsonValueReaderWriter<TModel>,
     IJsonConvertedValueReaderWriter
 {
+    private static readonly bool UseOldBehavior36856 =
+        AppContext.TryGetSwitch("Microsoft.EntityFrameworkCore.Issue36856", out var enabled) && enabled;
+
     private readonly JsonValueReaderWriter<TProvider> _providerReaderWriter;
     private readonly ValueConverter _converter;
 
@@ -52,8 +55,16 @@ ValueConverter IJsonConvertedValueReaderWriter.Converter
 
     /// <inheritdoc />
     public override Expression ConstructorExpression
-        => Expression.New(
-            _constructorInfo,
-            ((ICompositeJsonValueReaderWriter)this).InnerReaderWriter.ConstructorExpression,
-            ((IJsonConvertedValueReaderWriter)this).Converter.ConstructorExpression);
+        => UseOldBehavior36856
+            ? Expression.New(
+                _constructorInfo,
+                ((ICompositeJsonValueReaderWriter)this).InnerReaderWriter.ConstructorExpression,
+                ((IJsonConvertedValueReaderWriter)this).Converter.ConstructorExpression)
+            : Expression.New(
+                _constructorInfo,
+                ((ICompositeJsonValueReaderWriter)this).InnerReaderWriter.ConstructorExpression,
+                // We shouldn't quote converters, because every new instance will have to compile
+                // the expression again and it will have a negative performance impact.
+                // This means this is currently unsupported scenario for precompilation.
+                Expression.Constant(((IJsonConvertedValueReaderWriter)this).Converter));
 }