If you're really performance obsessed, a spreadsheet isn't really the best solution anyway...
I agree with this, but tell that to the millions of non-programmers the world over (including almost exclusive use in the finance sector) who use spreadsheets for, well, programming.
I have heard it's not unusual for huge spreadsheets that take upwards of 15 minutes just to regenerate all the calculations and VBA macros it contains. Yes, this is horrible, but it's absolutely ubiquitous and so performance does matter. VBA support also matters for backwards compatibility with these monolithic, battle-tested behemoth spreadsheets. Python support is a great extra, but drop-in VBA support is the feature Gnumeric needs to find widespread use.
I can't fathom what you mean by "more accurate". Accuracy is binary. Are you saying Excel is inaccurate?