What are you trying to countersink these holes in? Wood, aluminum, steel? Are you using any coolant? 12mm is a pretty large diameter ; normally it would want to be run slower than that, but it should be okay if it's just wood. If your spindle is slowing down under the load, you might be pushing it too hard. Also, as you're already finding out, crappy tools don't work as well as good ones - sometimes they don't work at all.