The UM3 head is a bit heavier than the UM2(+) head. As far as I recall they tuned the acceleration & jerk settings to compensate for this.
You can change these settings in Cura if you want to. We tune settings for generic cases, so it's possible to create better settings for specific cases.
This is probably due to the acceleration and jerk settings which can be so low it will never get to full speed.
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