[ken]

Would this hypothetical fix include a performance suite, and a regression testing system, to ensure this task stays fast forever?

IME, systems that don't focus on performance as a primary metric tend to slow down over time as they accumulate features. Someone who is willing to buy Excel ($130) may not be willing to pay a $100 bounty every time some other programmer makes LibreOffice slower again.

I googled for LibreOffice performance regressions and found several, going back many years, and they tend to include a one-line fix. For all its faults, I'm sure there's a team at Microsoft that knows every time Excel performance regresses.

[mariushn]

Good point. How can one prevent this though, for any free/open source software? Introducing issues (intentional or careless) to keep some funding going.