The speed of the aircraft is somewhat irrelevant. The components inside the engine can move at speeds of 1800 kph while the plane is standing still. In the fan and compressor sections of a modern engine, abrasives like ash will erode material, making the aerodynamics worse and possibly causing liberation of moving parts. Once the ash makes it past the compressor, the 2500 F firing temperature melts the ash, which then deposits on the turbine section, causing poor aerodynamics, vibration, and other not so pleasant outcomes.