Yep, the bigger cutter only has just under 5mm spacing between the steps so in 10mm, 3 cutting edges are working all at once for a moment then it's back to two. I can't see why that is the limiting factor.
It either works or it does not.
Maybe it's your cutters or sharpening? I can't say the step drills I have are top of the line ($45 irwin 3 set) I can't fault them considering what I put them through.
I just use a coarse diamond stone and touch up the leading face from time to time with good results