We suggest you to add the below code snippet in your sample so that the loading time will be reduced while doing any drag and drop operations.
this.olapClient1.OlapChart.OptimizeLargeDataLoading = true;
Enable the OptimizeLargeDataLoading below the InitializeComponent() method ie., after initialize all the components only the olapclient can activate OptimizeLargeDataLoading property.
Please let us know if you have any further concerns.
While cube is too large, data needs to be extract from the cube obviously will take more time. At such circumstance we can use the virtual scrolling option to avoid this problem so that the data would be viewable page-wise.
Please find online sample demo in following link for virtual scrolling.
Please let us know if you have any concerns.