Router melt marks appear as smeared edges, rounded corners, strings, or chips welded back onto the cut. Frictional heat is being generated faster than the cut and airflow can remove it.
High spindle speed with low feed is a common combination, but cutter geometry, dullness, flute count, depth of cut, part temperature, and weak chip evacuation also matter. A flexible part can vibrate and rub instead of cutting cleanly.
Correction
Feed and speed should be developed as a pair for the material and cutter. The tool must produce chips rather than dust or a continuous melted ribbon. Airflow or extraction should clear chips from the cutting edge without contaminating the surface.
The part should be fully supported and sufficiently cool before routing. Increasing feed alone can overload the cutter or fixture, so spindle load, edge quality, and dimensional accuracy should be monitored together.