Why is Excel suddenly calculating much slower than before?

I've run into this a few times. Before diving deep, I'd check whether the slowdown is specific to the workbook or to Excel in general.

A few common causes:

• Excessive conditional formatting (especially if applied to entire columns)
• XLOOKUPs referencing large ranges or entire columns
• PivotTables connected to large datasets
• Volatile formulas (OFFSET, INDIRECT, TODAY, RAND, etc.)
• Hidden sheets, named ranges, or objects that have accumulated over time
• Workbook corruption/bloat from years of edits, copy/paste operations, and saved history

One quick test is to save a copy under a new name and see whether performance changes. I've also seen cases where recreating the workbook structure in a fresh file significantly improved responsiveness.

Other useful checks:

• Set calculation to Manual and see if the lag disappears.
• Check Conditional Formatting Rules Manager for duplicate or oversized rules.
• Review Name Manager for obsolete named ranges.
• Check workbook size versus actual used range (Ctrl+End on each sheet).
• Try opening the file on another PC to determine whether the issue follows the workbook.

If the slowdown appeared gradually over weeks rather than immediately after a large change, I'd personally suspect workbook bloat, conditional formatting, or formulas referencing larger ranges than intended.