Not everything is re-rendered every time you add a post. Only a new or modified post will be rendered if you run blogdown::serve_site. See Yihui's response here
You can also customize your caching by post, or chunk, as described here (n.b. this is a link to the Different building methods section, the metadata just doesn't show up that way):
I'm not sure that these solutions necessarily fit your needs, but I'd recommend checking out the thread below, especially @andrewheiss' response.