It's difficult to comment without seeing your workbook. But broadly if you're developing any kind of useful analytical model you will find the performance fall

It's difficult to comment without seeing your workbook. But broadly if you're developing any kind of useful analytical model you will find the performance fall off a cliff very quickly if using 1 million rows. e.g. waiting around 25 minutes to save the file (or being corrupted), the software being suddenly very slow and weird. And this includes tricks like turning off automatic calculations, hard copy pasting in the results of calculations (rather than keeping all cells as formulas), only running calculations on specific cells etc. There are things you can do to try and manage big data in excel. But it becomes a chore and ultimately a big bloated monster. And why bother with the pain when you can use Python (e.g pandas) or R or SAS. Power excel / an SQL backend (with VBA) is also a common solution. Or was. I mostly work in Python/R these days.