diff --git a/docs/release-notes/.FSharp.Compiler.Service/11.0.100.md b/docs/release-notes/.FSharp.Compiler.Service/11.0.100.md
index 7dd68964cb3..97589376dc8 100644
--- a/docs/release-notes/.FSharp.Compiler.Service/11.0.100.md
+++ b/docs/release-notes/.FSharp.Compiler.Service/11.0.100.md
@@ -166,6 +166,7 @@
 * Add symbol and type highlighting to F# diagnostics ([PR #20097](https://github.com/dotnet/fsharp/pull/20097))
 * IL: add `ILPreNamespace`, make `ILPreTypeDef` creation lazy ([PR #20092](https://github.com/dotnet/fsharp/pull/20092))
 * IL: use empty tables for members when possible ([PR #20249](https://github.com/dotnet/fsharp/pull/20249))
+* IL: hold custom attributes in fields rather than a union case
 
 ### Improved
 
diff --git a/src/Compiler/AbstractIL/il.fs b/src/Compiler/AbstractIL/il.fs
index 0aa4e76ecf4..4d33e1ff78b 100644
--- a/src/Compiler/AbstractIL/il.fs
+++ b/src/Compiler/AbstractIL/il.fs
@@ -1262,14 +1262,18 @@ type WellKnownILAttributes =
     | OverloadResolutionPriorityAttribute = (1u <<< 26)
     | NotComputed = (1u <<< 31)
 
-type internal ILAttributesStoredRepr =
-    | Reader of (int32 -> ILAttribute[])
-    | Given of ILAttributes
-
 [<Sealed; NoEquality; NoComparison>]
-type ILAttributesStored private (metadataIndex: int32, initial: ILAttributesStoredRepr) =
+type ILAttributesStored private (metadataIndex: int32, reader: int32 -> ILAttribute[], given: ILAttribute[] | null) =
+
+    /// Stands in for the reader when the attributes are already in hand, so the field can stay non-null.
+    static let noReader: int32 -> ILAttribute[] = fun _ -> [||]
+
+    // Holds the array rather than an ILAttributesStoredRepr. The reader function is shared per metadata
+    // reader per attribute table, so it is held directly; wrapping it cost one object per owner, and most
+    // owners are never forced, so most of those existed only to say "not read yet". ILAttributes is a
+    // struct over the array, so rewrapping on each read allocates nothing.
     [<VolatileField>]
-    let mutable repr = initial
+    let mutable attrArray: ILAttribute[] | null = given
 
     [<VolatileField>]
     let mutable wellKnownFlags = WellKnownILAttributes.NotComputed
@@ -1277,12 +1281,12 @@ type ILAttributesStored private (metadataIndex: int32, initial: ILAttributesStor
     member _.MetadataIndex = metadataIndex
 
     member x.CustomAttrs: ILAttributes =
-        match repr with
-        | Given a -> a
-        | Reader f ->
-            let r = ILAttributes(f metadataIndex)
-            repr <- Given r
-            r
+        match attrArray with
+        | null ->
+            let a = reader metadataIndex
+            attrArray <- a
+            ILAttributes a
+        | a -> ILAttributes a
 
     member x.HasWellKnownAttribute(flag: WellKnownILAttributes, compute: ILAttributes -> WellKnownILAttributes) : bool =
         x.GetOrComputeWellKnownFlags(compute) &&& flag <> WellKnownILAttributes.None
@@ -1298,9 +1302,10 @@ type ILAttributesStored private (metadataIndex: int32, initial: ILAttributesStor
             wellKnownFlags <- computed
             computed
 
-    static member CreateReader(idx: int32, f: int32 -> ILAttribute[]) = ILAttributesStored(idx, Reader f)
+    static member CreateReader(idx: int32, f: int32 -> ILAttribute[]) = ILAttributesStored(idx, f, null)
 
-    static member CreateGiven(attrs: ILAttributes) = ILAttributesStored(-1, Given attrs)
+    static member CreateGiven(attrs: ILAttributes) =
+        ILAttributesStored(-1, noReader, attrs.AsArray())
 
 let emptyILCustomAttrs = ILAttributes [||]