Add perf explanation

src/Umbraco.Core/ObjectExtensions.cs

@@ -718,8 +718,10 @@ namespace Umbraco.Core
             if (AssignableTypeCache.TryGetValue(key, out bool canConvert))
             {
                 return canConvert;
-            }
-
+            }
+
+            // "object is" is faster than "Type.IsAssignableFrom.
+            // We can use it to very quickly determine whether true/false
             if (input is IConvertible && target.IsAssignableFrom(source))
             {
                 return AssignableTypeCache[key] = true;