to answer your questions:
Is it normal it slow down and hangs Jabref? For article item,
For the moment, it takes some time to generate the Preview style. This is already on our agenda to speed the generation of this up.
Also, you can we test those styles? I don't know any of those journals, so I have no idea what they look like.
You can test/get a preview of the citation styles here on the website of the project:
And also, how to create a second personalized preview?
This is the same as before, under Settings -> Entry Preview you just add the style "Preview" to the top on the right side and the custom preview will be used.
You can modify the custom HTML-preview in the "Preview"-Window in the settings.
A list of formatters you can use is described here:
For the speed up of the Citation Style, it is tracked here: