Excel 2007 : Macro - loop slows down

I created a macro that runs an iterative process. The macro basically makes some adjustments to some data and copies the data to a blank row. Then, the macro does the same thing over and over.

There are 13000 rows the macro cycles through and I noticed around row 4000, the macro's speed essentially comes to a stop. I also noticed at row 4000 my computer usage (via the task manager) jumps from 60% to 90%.

I assume it is not a coding issue since the macro runs for awhile, but then hits a wall in the middle of the iterative portion.

Does anyone have ideas as to what is going on?