I most live on Macs. But for big spreadsheets, my Microsoft Excel 2010 on a modern Windows PC is blazing fast to open, load, operate and calculate. Sub-400ms and 8.4MB RAM to launch.
Sadly, support ended October of 2020, so it's time to find something else before too long. It is not going to be contemporary Office.
If I have to believe this[1] thread, then Office 365 is now an Electron app, which would explain the slowness.
[1] https://news.ycombinator.com/item?id=17300893