I am using Safari 12.1.1 on macOS, with no extensions.
https://worldbuilding.stackexchange.com and https://anime.stackexchange.com seem to have a bit sluggish, inertial and non-smooth scrolling on their home pages as compared to the sites with no background image in the sidebar, such as https://apple.stackexchange.com.
Is that true or is my system at fault?
This screenshot was seen temporarily, but I do notice the sluggishness most probably due to the illustrations in the background.
The left part on the timeline is for Worldbuilding and right one for apple.
The blue marks are where I changed the web page.
Scrolled up and down a few times on per site.
This call tree screenshot is for Worldbuilding. I cannot find icon etc for export, thus the image.