From OpenSCADAWiki
Processing the widget in whole and executing its program performed at any event in the life cycle of the VCA project (specified in the "Period of the calculation" field of the "Project" tab, Figure 5.2) and periodically with the widget processing period (specified in the "Periodic processing" field of the "Widget" tab, Figure 5.1.3). By default, the widget processing period is taken from top widget and it is 1000 ms, so when you want faster updating your widget, you can set "Periodic processing" to 250 ms on the "Widget" tab.