a1.a188000 contain values.

I type =if(a2=a1,"dupl","") in b2
I copy that formula and paste it in B2.B188000
I select col b
I do FIND ALL dupl
It finds 12000 cells.
I press the Shift key and click on the first and last cells in the box
I do DELETE rows

Why does it take HOURS to delete the 12,000 rows?