Problem: Viewport Performance can be slowed down by a large number of polygons that were created with a Subdivision Surface Modifier. And going through and selecting object individually and turning down their level can take quite some time. And selecting all object and hitting Ctrl+# can add reduce the modifiers levels but at it to objects that should be smooth.
Solution: Have a Global Subdivision Surface Modifier that increase the level on all object that have a Subdivision Surface Modifier applied by either going up a step or down a step. The way this should work is by have these proposed keyboard shortcuts Ctrl+Shift+NUM+ to increase the level or Ctrl+Shift+NUM- decrease the level. This should be so that if there are object currently selected that they are the only ones affected or if nothing is selected than it should increase/decrease on every object with the modifier.