Google Sheets can become slow when thousands of cells recalculate at once or when a workbook relies heavily on imports, complex array formulas and repeated lookups.
Start with the obvious checks
Reduce formulas that process entire columns when only a smaller range is needed. Avoid repeating the same calculation in multiple places and consider helper columns for expensive logic. Review IMPORTRANGE, QUERY, FILTER and volatile functions such as NOW and RAND.
What else to check
Large numbers of conditional formatting rules, images and duplicate formulas can also increase loading time. If the sheet is part of an automated workflow, moving repeated work into Apps Script may be more efficient.
When to get help
Get help finding the bottleneck and restructuring the workbook.