Is is normal for the same process to take significantly longer each time it is repeated or is there something wrong with my computer?

My spreadsheet has 9 columns and 51,000 rows. It contains no macros nor formulas.

Here's what I'm doing:

I click on the "A" at the top of COL A.

In the FIND and REPLACE box, I type DUPL after 'Find What' and, for Options, I click 'WITHIN SHEET", "SEARCH BY COLUMNS" and "LOOK IN FORMULAS". I then click FIND ALL and it returns 588 cells.

I select the first cell listed and hold down the SHIFT key and select the last cell listed. (That selects all 588 cells listed).

I then click INSERT and INSERT CELLS and SHIFT CELLS RIGHT.

The first time, the "shifting" process" take 8 seconds.
the second time takes 20 seconds
the third time takes 80 seconds.
the fouth time takes 140 seconds
the fifth time takes 200 seconds
the sixth time takes 6 minutes.
the seventh time takes 20 minutes.