Relation between terminal voltage and charging speed in Lithium-Ion/Polymer batteries?
Have you read: https://batteryuniversity.com/learn/article/charging_lithium_ion_batteries ?

Look at a typical Lithium based cell charging curve:



Note how the charge level to voltage relation is very non-linear. When the battery is at a low charge the voltage increases more quickly with charge level compared to an almost full battery. So to get from 3.94 V to 4.20 V more energy is needed than going from 3.68 V to 3.94 V. So charging with another 10 W during one hour (1 Whr) will result in a lower voltage than 4.20 V