I have a spreadsheet with roughly 2000 rows and 18 columns. As part of a
macro, I sub total columns 7 and 17. I use a macro to copy select cells into
a second sheet.

After this, I remove the subtotals. I use lookups from the second sheet into
the first sheet. As part of my tests, I have since copied and pasted the
lookup cells as absolute values. There are no longer any lookups in any sheet.

The initial subtotal and it's removal, when creating the macro, took seconds
to complete, however on subsequent attempts, they take hours.

I have since copied and pasted the sheet into a different workbook, and the
subtotalling function in the second workbook takes seconds to complete.

What can I do to improve the performance?