Try manual unrolling

Author: am11

src/coreclr/System.Private.CoreLib/src/System/Runtime/CompilerServices/CastHelpers.cs

@@ -551,7 +551,26 @@ internal static object Box(MethodTable* typeMT, ref byte unboxedData)
             }
             else
             {
-                SpanHelpers.Memmove(ref boxed.GetRawData(), ref unboxedData, typeMT->GetNumInstanceFieldBytes());
+                ref byte dst = ref boxed.GetRawData();
+                nuint bytes = typeMT->GetNumInstanceFieldBytes();
+                switch (bytes)
+                {
+                    case 1:
+                        Unsafe.As<byte, byte>(ref dst) = Unsafe.As<byte, byte>(ref unboxedData);
+                        break;
+                    case 2:
+                        Unsafe.As<byte, ushort>(ref dst) = Unsafe.As<byte, ushort>(ref unboxedData);
+                        break;
+                    case 4:
+                        Unsafe.As<byte, uint>(ref dst) = Unsafe.As<byte, uint>(ref unboxedData);
+                        break;
+                    case 8:
+                        Unsafe.As<byte, ulong>(ref dst) = Unsafe.As<byte, ulong>(ref unboxedData);
+                        break;
+                    default:
+                        SpanHelpers.Memmove(ref dst, ref unboxedData, bytes);
+                        break;
+                }
             }
 
             return boxed;