I’ve been trying to do some testing in there and I’m not quite sure what actually the problem. Because sometimes the customizer works fine and sometimes it’s doesn’t load properly. Also, I don’t get any error message in there that causing the customizer doesn’t work properly in there. As far I know, this problem related to the memory usage, wp memory limit, and PHP memory limit. In this case, may I know the specification of the server that you use in there?