If I summon a wither skull with {power:[0.0,0.1,0.0]} then it smoothly moves upwards.

But, if I summon a wither skull with {power:[0.0,0.0,0.0]} and then change it to {power:[0.0,0.1,0.0]} with the /data command, the skull is lag moving upwards.

As of 1.15.2, this happens even upon initial summoning. Use this command while at least 65 blocks above ground level and observe the skull make its way upwards:

/summon minecraft:wither_skull ~ ~-60 ~ {direction:[0.0,1.0,0.0],power:[0.0,0.01,0.0]}