Excel file too large to work?

Check for volatile functions such as OFFSET(), INDIRECT().

TODAY() in a copied formula can really drag down performance.
Better to put =TODAY() in a fixed cell, say $A$1, and refer to that rather than build it into a formula.
e.g. in A2, Dragged Down

Formula:

=$A$1+ROW()

Not

Formula:

=TODAY()+ROW()

Conditional formatting applied to whole Columns can also be expensive, C/F is also volatile.