Some answers as a user:

No scripting language, most of the heavy lifting done in plugin is through shell calls (asynchronously or not). Example here [1]. I personally think that it's kinda hard to grok to be honest.

Thanks to the client server architecture, it is possible to make an external UI[2].

There is no line wrapping currently. Don't know about long documents, but the editor in general is very efficient.

[1] https://github.com/mawww/kakoune/blob/master/rc/core/grep.ka...

[2] https://github.com/mawww/kakoune/blob/master/doc/json_ui.asc...