tempranova

This is a simple plugin that uses AJAX to load comments, converting the “Newer/Older Comments” button into a button that loads more comments instantly.
This plugin also paginates nested replies. You can customize styling in whatever way you choose.
Using Settings > Comment Load More, user can set desired pagination (currently both comments and replies have the same number).

Comment Load More

This plugin allows you to load more comments instantly via a button push (AJAX), instead of pagination that reloads the page.

Mark Jaquith

When your posts get a lot of comments, performance can be a concern. The generation of the comment HTML and the delivery of all that HTML can slow down your page loads. This plugin has pages load without comments, and then uses a secondary AJAX request to pull down the comments. Navigating through the pages of comments is also handled via AJAX. On my test install with 100 comments per page, the <code>onload</code> event fires 2x faster. The HTML is generated by the server 6x faster. The HTML downloads 3x faster. The uncompressed size of the HTML is reduced by a factor of 17.
Initial development of this plugin was commissioned by Google — many thanks!

AJAX Comment Loading

Loads comments with a secondary AJAX request, including comment paging. Speeds initial loading of the page.

Comment Load More vs AJAX Comment Loading