A few dead keys on a MacBook can come from an app, Accessibility setting, debris, or a failed key mechanism. A key-by-key pattern is useful: one sticky key is a different diagnosis from a whole keyboard that is unresponsive.
A laggy iPhone keyboard can be caused by a frozen app, low storage, a third-party keyboard, or broader performance trouble. First compare the symptom in more than one app so you do not reset the phone for an app-specific problem.
An iPad keyboard may connect through Bluetooth or through a Smart Connector, and each route has a different failure pattern. Check power and pairing for Bluetooth models; check contact alignment and cleanliness for Apple’s connector-based keyboards.