How to organize multi-level comments with pagination?

Question

PHP

Site on 1C-Bitrix. I am planning a functional comment on the site. The number of comments to one element can reach several thousand. Ready-made solutions from the marketplace often load the server very heavily, and due to some specific requirements they are not suitable for the current project.
The comments use parent, which contains the id of the parent, to form the tree. How can I generate a query in the database with nPageSize so that I can’t figure out the pagination of the comment? If you set nPageSize, then you will not be able to display all nested comments. Nested queries are also not suitable, because create a painful load on the database.
Prompt, please, resource-saving approach to the given task.

Answer 1

Load attachments via AJAX?
On demand or after the page has loaded.

Answer 2

www.realcommenter.com
Specially launched a test subdomain, where a new portion of comments is regularly generated in the machine.
There are already 1,800,000 comments
test.realcommenter.com