Customize field: content lost at save / the onload appears late

I add a field using customize form (I associated it with a Jquery widget). Then I add some custom script.
First of all, the form loads very slowly (the onload) is called very late and very irregular.
sometimes before:
localStorage cleared
Cleared App Cache.
localStorage cleared
and sometimes after.

error of type
Source map error: request failed with status 404
Resource URL: http://localhost:8000/assets/js/libs.min.js?ver=1521424459.0
Source Map URL:[Learn More]

appear with unknown reason.

and even worse, when I save it wipes off content of my field.

Any help is highly appreciated.

Kind regards