Classic Outlook Keeps "Not Responding" and freezing

Are you using it  in the cloud or locally? One Drive lacks the memory to make alterations to large spreadsheets. Open it locally and try again.
Check the extent of your data. Start at the top and press CTRL + down area. Does it stop at the last cell or continue? If so your data contains non printing characters. Since Excel processes every cell,, it overwhelms the available memory. There are a number of complex ways to remove them, but the easiest is to copy a seemingly empty cell and paste it into find. Put a character group you don't expect to have in the rest of the sheet in the replace line (vvv, for instance), and replace all. Then find vvv and leave the replace with space empty, replace all again.
Try removing all conditional formatting. (Home>Conditional Formatting>Clear rules.