>All other IDE features like auto-completion happen asynchronously

in the technical sense, but you as a developer don't use auto-completion asynchronously. It's not like you autocomplete and continue typing and then come back to the completion. When you complete at point you have wait. Whether that keypress takes 2 or 3 milliseconds isn't going to make a difference when the inter-process communication of your editor and its services is magnitudes slower. It's not like programming is like playing an FPS game. You're not in any meaningful sense limited by your mechanical input speed.