On a Geeetech A20M (which has a bowden extruder), I was pulling my hair out over blobbing, maybe zits showing up where the print head would move from an inside perimeter to the outermost perimeter. I tried about 20 different combinations to lessen it.
Then, I lowered the "Default printing speed" to fix something else and raised the outline underspeed percentage.
I believe that by bringing the two speeds closer together, the blobbing, maybe zits were greatly reduced and almost eliminated.
If I had to guess precisely why, I would think it is probably because of the way the hot end on this machine is set up, as well as the bowden extruder. With a bigger delta between the two speeds, there was a bit of pressure built up that could to be relieved simply by retracting, especially given the bowden extruder which brings in an element of "lag" or "delay" to anything that goes on.
Hopefully this post helps someone else.