I suppose you refer to the page "Fast drawing" and press the "Add" button to add 50,000 data points. Then you zoom (this is per Shift-Left-Drag here, not the usual Left-Drag!). For me (Win 10, Laz 2.0.2, FPC 3.0.4) there is immediate reaction, no delay.
Please note that the ZoomDragTool becomes active in the MouseDown event but requires here the Shift key to be pressed also. Therefore, you must hold the Shift key down before you drag the zoom rectangle. Moreover, the zoom rect must be dragged in top-left to bottom-right direction although the vertical drag bars seem to indicate that this might not be required here. Otherwise there is no zooming at all. But I guess this is not your problem because you do see zooming, but delayed by a few seconds - right?