For deep slots like this I find it best to first drill multiple holes through the entire slot. The holes are spaced very close together and the edges of the holes will actually overlap. It is then much less work for the mill to clear out the remaining metal. My mill is a small one so the drilling out as much metal as possible really speeds up the process and reduces the wear on the end mill. Once the holes are drilled the end mill is used by stepping down about .050" and clearing the slot. Passes are made until the slot is cleared but rough. Finish passes are then made full length on both sides of the slot.

Cheers, JR