Consider this an mental exercise. If you have two pages which look the same and load at the same speed - view you save each page as a.html and b.html and upload them to the same location, they will perform the same on ranking, regardless of whether Jekyll or WordPress.
The downside with Jekyll is that the static content is based on commits, and doesn’t come with a admin view.
This can be overcome by using a CMS like Forestry.io so you have a admin interface that feels a bit like wordpress so you can update your content as commits (rather than pages in a database) and get your site built in the background. Jekyll Admin is also a neat plugin, for local use only though.
If your side needs thousands of users to upload content or to show that content in real time, then that would be bad fit for Jekyll as it is a static site. A site like Facebook is built on PHP and React I think so it let’s them make the pages dynamic and allow interactivity.
You can of course add React to your Jekyll or Wordpress page, to make a form more interactive or use plain JS pull in say stock price dynamically in real time without a page load.
With Jekyll, building the site might take a few seconds or a minute for a larger site. But the benefit is doing the work up front and then removing Jekyll from the serving of the content so a site with 1000 pages and 10,000 visitors will be super fast while a Wordpress site like that might take strain having to render each page from scratch on every page load on requested URL. You can of course adding caching to keep the pages faster to load and only use Wordpress on the initial load.
And a way to solve performance on Jekyll builds if that matters to you (especially local builds that take 1 or 2 minutes for 100s of pages like it can happen to me) then Jekyll 4 and its built in build cache. That feature allows you to say change one page and then rebuild the whole site, but it will only take like 1 second, because the other 99 pages are cached.
Also worthy mentioning is that there are a ton of static site generators that are built on React or Vue (JS) or Python or Go. Notably, Hugo is in Go and supposed to be the fastest.
But I find Jekyll brilliant for most of my needs so I build my static sites with Jekyll.