First I want to say you have done a great job with the customizer. Further I must say that I only have plus-plugin to support you. I don’t really use it.
The loading time of the customizer is a long. For my old computer it is not long enough to catch a cup of coffee. But that is only because I have to go two stairs down and up again. With the new computer it’s a lot beter. But even then it is long. WP 4.4 brought a considerable beter performance. But still I have to count to 10 before I can go any further.As far I understand the loading time of the customizer depends on the possibilities one haves. As I’m not using all possibilities it would fit my to have in the customizer only those of the freeware, Regarding this it would be nice if there would be a possibility to choose only for the basic settings in the customizer.