After a year or so I migrated to a 5 USD droplet on Digital Ocean (back then GH Pages didn't offer https for custom domains) and integrated with Github webhooks to automate the deployment when pushing new markdown to the main branch.
Over the time it indeed started to degrade. The build time takes almost a minute, but after building the website is just a bunch of static html pages.
Nowadays it is annoying to write new posts because I like to write locally and refresh to browser to check if it's looking good or not. So I would say it degraded for me but for the reader it's still as fast as it was when there was just a couple of posts.
I thought about migrating the blog to something else, but because I used some custom markdown extensions for code highlight and other things, it would be painful to migrate all the blog posts. So I've been postponing it since 2019.