@Stennie_X I had another test about the slow experience.
I had to reinstall (again) my system for an account sync problem; easier than trying to fix 
This time I had no extensions installed. Yet again, other sites were fine while the Forum was still slow.
After a few minutes, I noticed the pages were partly fast. Then I opened the browser’s task manager and saw a high CPU usage.
The usual workings of web pages are to “load and keep” on page load or after scroll, but it seems it is like “load then forget and reload” for every scroll action.
Although it was initially worse just like I described in the first post, it gradually become faster to scroll.
This new video shows an increase in CPU usage while scrolling: mongodb forums high cpu usage.webm - Google Drive
These high levels are the culprit behind. It lowers the browsing experience on mid-to-low-end devices.
I hope web devs will find the actual code causing this and fix it.