Have you ever wished Excel could calculate faster — maybe even use your GPU?

There are lots of things in a spreadsheet that can cause it to slow down.  I'm not saying there is a cure for your issue because Excel is NOT a database solution but here are a few things to check and try:

a) use the (new) built in feature called Check Performance (go to 'Review' tab).  this feature will find numerous things in a workbook that can be causing slow downs

b) avoid use of excessive conditional formatting. that could be rules applied to huge ranges or just tons of rules

c) avoid use of 'volatile' functions.  These include functions like INDIRECT, OFFSET, RAND, etc... 

d) MAKE use of new functions like XLOOKUP and XMATCH instead of the older versions as they have improved performance (and functionality) on them

e) MAKE use of array calculations instead of individual calculations that are then filled down/right

Additional tips are much harder without seeing the actual book.  There are many threads here dedicated to or at least include discussion about improved performance.  For complicated formulas, it appears there can be significant improvement using a concept of THUNKS (a form of using LAMBDA functions)

hope that helps