If you are using a microflow to display a page, you can edit the microflow settings to show a progress bar
Preloading is not available to my knowledge, but you can get something like it by adding some widget (datagrid for instance) with the widget-setting visibility to conditional (to some condition you can change) and with the widget-setting datasource to microflow and make the microflow have setting Asynchronous and non-blocking. The data will get loaded without you having to wait and not shown directly.
Then you can add a button that will change the value of your set visibility-condition and showing the prefeteched data.
This is not exactly a preloading screen, but might help in the user experience for slow(er) loading screens: