So, using the yeelight app, or using the python implementation made by Stavros - All the fades, including color flow, are non-linear fades, they seem to be sin-like, changing fastest in the middle, and slow at the beginning and end.
Is this an inherent issue with the way our eyes perceive colours, and that some colours are “slower” to change, when they are changing at the same rate, or is it just that there is a form of fading algorithm to make simple transitions looks smoother?
Thanks 
In our app, the mode color flow will be set same rate of every color.
and for your question, there is some reasons about human’s perception.
people’s eyes have different perception about different color, pure blue, green and red are more sensitive, so if a pure blue change to other color, we will perceive the change sensitively, but other color will be not, so that you may feel different rate.