The problem with spreadsheets is generally maintainability and testing not performance. At least if it is slow people might rewrite it on a better platform.
We're hoping that better performance will let users build well-designed spreadsheets - they'll be able to show more steps and use less archaic formulas without having to worry about slowdowns.
If you can speed up serious financial sector spreadsheets - hideous baroque constructions of formulae and VBA - they will pay you any amount of money for this.