Depending on the engine computer (or lack thereof), a couple seconds of throttle lag isn't unreasonable. One of the things my flight instructor made a point to show me while learning to fly small planes (with carbureted engines, so the effect was magnified compared to what you'd see on a modern fuel-injected auto) was throttle lag on a balked landing (go-around). Most aircraft engines directly drive the propeller, so there are no transmission effects to confusing things. Shoving the throttle from idle to wide-open can actually cause a dip in RPM as the higher manifold pressure condenses formerly vaporized fuel and the fuel system tries to catch up.