Engine time is the frame time not spent in events, physics or draw calls, i.e. just the overhead of the C2 engine. You should note that the measurements are all approximate, and 10% is still pretty low, so it may well be fine.
Engine time includes "ticking" objects, such as stepping sprite animations, moving any objects with movements, other behavior tasks e.g. searching for targets with the Turret behavior, and anything else that needs tick-by-tick attention by the engine that is not covered by events.