How many flutes and length of cut on the endmill?
Typically I use 6 inches a minute for a 2 flute and a .012 depth of cut leaving .002" for finish.
I would also maybe drill the slot out with a 3/64s if I was having trouble. If you are straight plunging I would slow that move down to 1.5.
Also using NX I can make the tool ramp on shape, kind of like thread milling into a hole.