Before the Blogger post editor "Jump Break" solution was provided, there were at least 3 known variants, for auto summarising posts. One was provided in Blogger Help: How can I create expandable post summaries?; and at least 2 others were provided as third party suggestions. It's possible that any of these 3 variants are present in some blogs, where the owner has not seen fit to remove them, in favour of deploying the "Jump Break" in new post editor.
Cloud Strife, in Blogger Help Forum: Problem Rollup: Auto Pagination for blogs with Layouts templates suggests an obvious solution - replacing use of all early "Read More" variants, with the new post editor "Jump Break".
The problem as I explained earlier is that with the older solution, the blog post still had all the content sent to the viewers' browser while just making the post look smaller by not showing all the content. This will solve the aesthetics problem but not the slow-ness problem. The perceived slowness differs much more to the person who has cached everything in the browser (mostly us who view our own blog) than those who view it first time or rarely.
The new feature which is as simple as adding an extra <!-- more --> in the HTML or rather simply using the new editor, keeps the look and feel as good and makes your website much faster to load. Which I am sure your readers will appreciate. And once the site loads faster for your viewers they are more likely to visit the pages you have there.
I'll add the advice that adding the "Jump Break" option to your posts will have to be done on a post by post basis, but the individual post update is fairly simple to do. And, taking the advice from Cloud Strife
with the older solution, the blog post still had all the content sent to the viewers' browser while just making the post look smaller by not showing all the contentit appears that not all "Read More" solutions actually reduce page size when loading.
The bottom line - if you replace the old "Read More" with the new "Jump Break", your readers will see a significantly faster main page download. Or, if you wish, you will be able to load more (abbreviated) posts total, and stay within Auto Pagination induced page sizes.
As Blogger continues to resolve the problems associated with Auto Pagination, its not a certainty that all 3 earlier used variants of "Read More" will continue to operate. If your blog uses an out of date "Read More" variant, and you're unable to replace it with the recommended "Jump Break", you might want to watch the display of your blog carefully, during the next week or so.