Have you tried some that don't work the way you want?
In terms of native, app user interface elements (like calculator buttons) and the math done by a calculator are all done on the watch. It would be very hard to make a watch OS (1 or 2) app that did not do this. Native app or not should not impact the calculator button response.
Maybe you are seeing slowness of calculator apps to load when you start them? That can be improved depending on how the developer designs the app.
PCalc is good. If you use RPN you may want to check out RPN Calc from James Mattis. James has a long history of coding HP emulators for smart devices. I have both and prefer RPN Calc for the iWatch. Unlike PCalc all of the basic functions are on the main screen. The tip calculations included with PCalc I can do in my head. RPN Calcs unit conversions are available on the watch, very handy. For RPN on the iPhone or iPad, i41cx+ rules.