Deleting objects through Schematic View Problem

When I try to delete multiple objects through Schematic View by something causes 3ds max too slow deleting process and I ever should to kill the 3ds max process to close and lose everything unsaved.
The solution is click on the scene viewport with middle mouse button click and will press delete keyboard and it done normality.

The map on scene are my Rio project from classic Driver 2 game.

Tip: On the scene viewport press middle mouse button click + alt and you can rotate the camera.