Performance

Although custom controls and user controls follow different authoring models and have different characteristics, both derive indirectly from the same base class, System.Web.UI.Control . A user control's implementation is declarative; however, it is parsed as a control and compiled into an assembly the first time it is used by a page. Thereafter, a user control behaves like any other compiled (or custom) control. As a result, there is no significant performance difference between the two models. Therefore, you should choose between the two models based on factors other than performance, such as deployment, authoring, and design-time support.