For what little it is worth, and it is presumably quite little as anecdata in present company, I've never noticed any performance issues with the Universal version of Calculator, and "time until first keypress recognition" feels the same to me as Windows 7's Win32 Calc.exe, with the old synchronous window message pump "wait for the hourglass to stop spinning" of a classic Win32 window replaced with a usually very quick splash screen.

It seems to me just as likely that y'all've forgotten all the time you've spent waiting for spinning hourglasses in Win32 (good old, rose tinted glasses phenomenon that) when you deride Universal app performance. Sure, there are plenty of Universal apps that could use a performance tune-up, in general, but there are just as many (or more likely plenty more) bad Win32 apps out there whose terrible performance is papered over with hourglasses and the passage of time afforded by Moore's Law.