Optimized response rendering time when no sparse fieldset is requested. This relies on the fact that the resource model is frozen after building the resource graph.

|                               Method |     Mean |   Error |  StdDev |
| ------------------------------------ | -------- | ------- | ------- |
| SerializeOperationsResponse (before) | 168.6 us | 1.74 us | 1.63 us |
| SerializeOperationsResponse (after)  | 148.5 us | 1.06 us | 0.99 us |
| SerializeResourceResponse   (before) | 123.7 us | 1.12 us | 1.05 us |
| SerializeResourceResponse   (after)  | 119.6 us | 1.05 us | 0.98 us |

This makes SerializeOperationsResponse 12% faster and SerializeResourceResponse 3% faster. What the benchmark does not show is the performance improvement on subsequent requests, so in practice the gain in higher.

Author: Bart Koelman

src/JsonApiDotNetCore/Queries/Internal/SparseFieldSetCache.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Linq;
@@ -12,6 +13,8 @@ namespace JsonApiDotNetCore.Queries.Internal
     /// <inheritdoc />
     public sealed class SparseFieldSetCache : ISparseFieldSetCache
     {
+        private static readonly ConcurrentDictionary<ResourceType, SparseFieldSetExpression> ViewableFieldSetCache = new();
+
         private readonly IResourceDefinitionAccessor _resourceDefinitionAccessor;
         private readonly Lazy<IDictionary<ResourceType, IImmutableSet<ResourceFieldAttribute>>> _lazySourceTable;
         private readonly IDictionary<ResourceType, IImmutableSet<ResourceFieldAttribute>> _visitedTable;
@@ -118,27 +121,37 @@ public IImmutableSet<ResourceFieldAttribute> GetSparseFieldSetForSerializer(Reso
 
             if (!_visitedTable.ContainsKey(resourceType))
             {
-                IImmutableSet<ResourceFieldAttribute> inputFields =
-                    _lazySourceTable.Value.TryGetValue(resourceType, out IImmutableSet<ResourceFieldAttribute>? fields)
-                        ? fields
-                        : GetResourceFields(resourceType);
+                SparseFieldSetExpression inputExpression =
+                    _lazySourceTable.Value.TryGetValue(resourceType, out IImmutableSet<ResourceFieldAttribute>? inputFields)
+                        ? new SparseFieldSetExpression(inputFields)
+                        : GetCachedViewableFieldSet(resourceType);
 
-                var inputExpression = new SparseFieldSetExpression(inputFields);
                 SparseFieldSetExpression? outputExpression = _resourceDefinitionAccessor.OnApplySparseFieldSet(resourceType, inputExpression);
 
-                IImmutableSet<ResourceFieldAttribute> outputFields =
-                    outputExpression == null ? GetResourceFields(resourceType) : inputFields.Intersect(outputExpression.Fields);
+                IImmutableSet<ResourceFieldAttribute> outputFields = outputExpression == null
+                    ? GetCachedViewableFieldSet(resourceType).Fields
+                    : inputExpression.Fields.Intersect(outputExpression.Fields);
 
                 _visitedTable[resourceType] = outputFields;
             }
 
             return _visitedTable[resourceType];
         }
 
-        private IImmutableSet<ResourceFieldAttribute> GetResourceFields(ResourceType resourceType)
+        private static SparseFieldSetExpression GetCachedViewableFieldSet(ResourceType resourceType)
         {
-            ArgumentGuard.NotNull(resourceType, nameof(resourceType));
+            if (!ViewableFieldSetCache.TryGetValue(resourceType, out SparseFieldSetExpression? fieldSet))
+            {
+                IImmutableSet<ResourceFieldAttribute> viewableFields = GetViewableFields(resourceType);
+                fieldSet = new SparseFieldSetExpression(viewableFields);
+                ViewableFieldSetCache[resourceType] = fieldSet;
+            }
 
+            return fieldSet;
+        }
+
+        private static IImmutableSet<ResourceFieldAttribute> GetViewableFields(ResourceType resourceType)
+        {
             ImmutableHashSet<ResourceFieldAttribute>.Builder fieldSetBuilder = ImmutableHashSet.CreateBuilder<ResourceFieldAttribute>();
 
             foreach (AttrAttribute attribute in resourceType.Attributes.Where(attr => attr.Capabilities.HasFlag(AttrCapabilities.AllowView)))