You should know that, we don’t need to open many customizer tab, but we need a stable theme code which will not cause our serve stuck.
At the beginning, we discovered our server will get stuck with unknown reason. You know, such issue is not acceptable for any website.
We contact you about that but got no good answer. After that, we keep encounter server stuck problem sometimes. Fortunately we finally narrow down the condition, do the test and give you the detail and confidently say this is something related with your theme, and highly related with your some code hooked in wp-customize.php.
And we are still not sure if such code are hooked else where in other public webpage. So we need you to look on this and give us a clear guide and information to prevent server get stuck with your code.
We are still looking forward to a good information on this issue from you, Thanks