Usually, our spreadsheet can load up to 10k issues in less than 5 secs. If you are stuck on the loading page, there might be a cache preventing you open the spreadsheet. One quick way to resolve this is by clearing your web browser cache.
Test with a Private Browser
A website can store cookies, caches, and other data on your computer. Issues with that data can affect your use of the website. To prevent the website from using that data, view it in a private window.
We have known problems with loading Excel-like Issue Editor with 3rd party apps or scripts as follows:
draw.io Diagrams for Jira
Asset Tracker for Jira-Asset Management