That will not be because of an array data structure for the lines. When properly done, plugins/extensions are in separate process which do not hang the main UI

That will not be because of an array data structure for the lines. When properly done, plugins/extensions are in separate process which do not hang the main UI thread. It helps if these processes has a low priority. Also, ionice is good when processing many documents. Another flaw of many extensions/language servers have, is the lack of abort controller implementation. This means that debouncing doesn't work. So if there are many document updates, the server would hang.