There are a number of things that can cause an Excel spreadsheet to slow down. Sometimes, the spreadsheet can become unusable, requiring up to a minute for saving or even the most basic of changes. Here’s a brief checklist of the most frequent causes of Excel slowdowns.

(Have a spreadsheet slow-down that I haven’t described below? Let me know in the comments! Thanks! -Rich)

• Network problems. If the spreadsheet is accessed over the network (local or remote), network issues can delay saving.
• Macros/VBA code. If you are using macros (or Visual Basic for Applications), make sure you don’t have unnecessary macros running, or macros that perform unnecessary calculations.
• Burdensome animation. Try disabling animation. 2003: Tools > Options > Edit > Turn off “Provide feedback with animation”. 2007: Excel Button > Excel Options > Advanced > General > Provide feedback with animation (uncheck)
• Complicated calculations. If you’re doing lots of calculations, you can disable auto-updating of these calculations. When you do so, your spreadsheet will not update until you explicitly desire (e.g. when you hit the F9 key). To disable auto-updating in 2007, go to the Excel Button > Excel Options > Formulas > Workbook Calculation, and select “Manual”.
• Duplicate Conditional Formatting Rules. Conditional formatting can be a processor-intensive task if your rules aren’t set up right. Manage your conditional formatting rules (2007) via the ribbon at Home > Conditional Formatting > Manage Rules… Try to consolidate similar rules into one rule (e.g. a rule for an entire column instead of a rule for each cell in the column)
• Funky Formatting. Formatting can really slow down a spreadsheet, especially on slower machines. Try removing all formatting: Removing All Formatting.
• Unnecessary junk. Sometimes, an Excel file can become backlogged with a bunch of unnecessary junk. Try saving as a CSV (comma-separated) file, which will strip out everything except for the data. (Make sure to not save over your original file in case this workaround does not, well, work! )
• Unknown, but it seems to help sometimes. Install the Lookup Wizard add-in.
• Other issues. Do you know of another issue that causes Excel to slow down? Let me know in the comments!

Assorted helpful links:

• Efficient Excel Spreadsheet Design