It's all about how you approach the job with high speed spindles. If you use 3 flute roughing cutter you can hog away big lumps at good feeds then finish with full depth pass. Works out quicker and cheaper because less wear on tooling.

Now to the OP your always going to struggle with 4 flute cutters with such low feed rate. Know this is going to sound crazy but needed to be more aggressive not less.! . . You really should have been double the feed.