(note, you can test the spreadsheets by continuing through the login page at https://my.blackbear.app as an anonymous user)
We're not necessarily aiming to be the fastest application in the world; find/replace is probably still faster to just do sed in the terminal. But if you're looking to do something like sort, filter, actually engage with the tabular data, then we can handle hundreds of thousands of rows --- files that excel won't even open.
I'll take a look at blackbear. One core difference I see is that we're a native app, fully offline. There is a single handshake with the licensing server on app start, but it's optional. License validation is handled fully locally, and missing network access is non-blocking for any features.