In addition, the reason why Application.Screenupdating=False helps "optimize"
the performance  of the underlying VBA code,
is because there are no refreshing activities being applied to the screen.
Another statement  in Excel's environment  to help
"optimize" the execution of the underlying code is
Application.Calculation=False, however it is VERY difficult in knowing
when & where to precisely place this statement, because in NOT knowing exactly
when and which specific Excel formulas
in a spreadsheet  ARE being executed at the precise moment within an underlying
VBA subroutine is being executed...