I don't think it is just the feed and speed, since the X axis direction is fine at the same speeds. I suppose increasing feedrate might reduce the need for better extraction though? If I push feedrate too high without increasing RPM of course the bit will deflect or break, it is only 3mm dia.

I can probably improve dust extraction (for one, I emptied my vacuum!). I'll have a play and let you guys know how I get on.