I have just started having the same problem. I have never had this problem for this week. I have been working on the same files for weeks and did not add any complex formulas.

When I start Excel, it is fine for about 15 minutes and then inserting or deleting row causing the file to slow down to a crawl. If I close and reopen, I it is fine again (for about 15 minutes).

The only thing that I might caused the issue is that the file went out and came back from someone using MAC. I resaved the file in my version of EXCEL.
It there such an issue regarding files going back and forth between WINDOWS and APPLE (MAC).

Thanks.

Jay