Preloading charts as soon as you enter a slide can significantly increase the performance around the selection of charts. However, if a slide contains a large number of charts, accessing this slide might take a moment.
If an element is placed above a chart, it can only be used if the preloading is disabled. Otherwise, the element
disappears in the layer behind the chart and can therefore not be used or edited.
The preload function can be disabled and enabled for a single chart via the eye symbol next to the upper right
corner of a chart.
This can be useful if, for example, you want to place a shape above the chart to add information in form of text or to highlight something.
To do so, follow the following steps:
1. Navigate to the user settings and globally enable the preloading of charts.
2. Select a chart and hover over the chart. The eye symbol appears next to the chart 
3. Click on the eye symbol to disable preloading. Figure 22. Disable Preloading
To enable preloading again, select the chart and then click on the eye symbol in the action bar
Down the section about preloading charts from the empower enterprise version here:
Please note not all features in the enterprise version of empower are available in express