Consider typical Medium blog post.
If by chance you block certain actions, you'll see it spin ridiculous amount of CPU, trying to sent tracking data all the time over graphql.
Some tracking systems injected (often without developer involvement, using so-called tag managers) will send your mouse position and clicks.
Through tag managers, you can easily end up loading 50 different scripts, often compiled with attendant framework code, usually in chains of tag manager loading tag manager (classic example - using Google Tag Manager to load Facebook manager and Google Ads, which then issue multiple calls to load JavaScript ads and trackers, which might involve further script loads).
And the tag managers are often managed by marketing in complete separation from any development or QA, so what was reasonably good website loading fast even under limited network conditions, suddenly turns into huge freeze-fest as 3 autoplaying videos get preloaded, 2MB JS/CSS animated overlay ad loads in, 3 ad boxes are filled in dynamically , and don't forget 15 trackers, 4 of them from ad services, 8 of them added by marketing team with possible duplicates, and 3 of them part of malware loaded by the ads.