Routing with 25mm pine

have just started to work with pine, mostly 25mm pine. I am making fairly detailed 2d cuts and using a 4mm drill (cant use much thicker drills), 16,000 rpm and 1200 feed rate. I need to find out how many passes are necessary, it has been suggested that 5 passes will be required for 25mm wood making but this really makes it a slow process. I know I can increase rpm and feed rate but this my starting point which I will hope to increase as I keep experimenting (have already snapped a few drills so need to be more careful now.)
Any suggestions?
thanks