I think you need to approach the site structure a little bit differently, because that many subpages (articles) per page (blog) is just very much for a file-based system in general (not just for the Panel).
The rule of thumb for Kirby subpage performance is: If you know that a page will have very many subpages (how many depends on the site, server and type of page) and you can restructure the content so that the many pages are split up into multiple containers, do that.
Example for your use-case: Create a subpage per year so that “old” blog posts are archived away to the year’s subpage. This should make your site perform much better as Kirby doesn’t have a problem with many subpages, only with many subpages per page.