Wow, I had never imagined this would be the culprit.
I have identified at least one of these image files: dark-bg.png (4098×10726) (mdb-community.s3.amazonaws.com)
In a 10s page performance test, almost 50% was spent in the page painting stage. I have added it to my blocked URL list and cleared the cache for the forums. now pages flow blazing fast compared to the previous state.
Files with 500kb sizes are generally just big in disk size, but this guy is in png format and can occupy a lot of CPU resources to display. and the forum pages try to refresh for every scroll event.
@Stennie_X, can you please warn the responsible team? (by the way, did the forums disable the tagging/viewing MongoDB team members?)