I have been developing extremely large spreadsheets (>700 Mb, now approaching 1Gb) in Excel 64 to simulate mineral treatment processes. I successfully developed one large spreadsheet of this nature late 2018 and had few issues with copying the model from 10,000 rows to 400,000 rows (the model is about 200 columns wide and each row represents a minute of elapsed time). I now have a model that is >1000 columns across but it takes forever (>18 hours) to copy the model from 1000 rows to 20,000 rows. The problem occurs on the same laptop used to develop the first model (gen 5 i7 chip with 16 Gb RAM) and on my new laptop (gen 8 i7 with 32 Gb RAM). Is there a known problem with mega-spreadsheets when they get too wide?