People are talking about "lag" and performance, as if this was a "the system can't handle it!" issue. This is a misleading description. The actual issue is that some developer at Apple screwed up their animation configuration when implementing the calculator redesign, and nobody tested adding more than two numbers together quickly. Multiple failures, none involving system performance.
What's happening is: the operator buttons have an animation on them, and they don't register inputs while the animation is running. The animation is pretty long, so it's quite easy to tap an operator, tap a number, then tap the operator again and have your input be missed.
Conveniently, this should be a trivial fix. Just abort the animation when a second input is received while it's still running and register the touch. (Or, lazily, make the animations a lot faster.)
Edit: I got operator/operand mixed up while writing this. :P
What's happening is: the operator buttons have an animation on them, and they don't register inputs while the animation is running. The animation is pretty long, so it's quite easy to tap an operator, tap a number, then tap the operator again and have your input be missed.
Conveniently, this should be a trivial fix. Just abort the animation when a second input is received while it's still running and register the touch. (Or, lazily, make the animations a lot faster.)
Edit: I got operator/operand mixed up while writing this. :P