Speed related artifact

Hello,
Can someone help me with this problem?
I am printing this in vase mode one wall, bottom part is printed at 120mm/s 3000 acceleration,
upper part is printed at 170mm/s with 3000 acceleration.
What would cause this difference in print ? I initially thought it is the belt rubbing somewhere but it seems as it is speed related.
both stepper motors are 1 month old, ldo 42sth48 2504ac running stock uncooled settings
IMG_2921.jpg
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