Most of the existing SEO plugins add some processing and requests to every page load, but it usually isn’t that bad.
In any proper setup, caching plays an important role in processing the page generation ones (PHP evaluation + the corresponding database queries to pull the right data based on user capabilities, taxonomies, URLs, meta entries, etc) and then adding it to the cache pool so it renders directly over a period of time (assuming that changes don’t happen all the time).
There are some exceptions here—most notably for logged-in users, especially in eCommerce environments, but this is the general process that the majority of the WordPress sites use (especially the more content-heavy ones).
However, you must know that some plugins like Yoast add tons of logic (the plugin itself contains 1,300 files) so there are loads of features that could be incorporated additionally. Luckily, most of them are processed and loaded on the backend—primarily in the dashboard area, checklists for readability or SEO analysis, tracking target keywords, suggesting internal links based on your database archive.
In some cases, all of this indexing happens in the background, and larger websites may see some unexpected spikes or hanging CRON jobs in the middle of the night if something happens.
It doesn’t happen often, but we’ve had similar edge cases with different SEO plugins on the market. Under normal circumstances, the installation itself works fine, but if background processes run frequently, they may impact the load times or accessibility even if a logged-in user isn’t interacting with the site.
Do you want to optimize the speed of your website? I have written about 14 Ways To Dramatically Speed Up Your Website From A Technical Perspective which can help you ensure a faster and better user experience of your website. Check out the article now!