No, it has nothing to do with loading performance (internet bandwidth). In Chrome, it’s instant, in plugin, it’s literally unusably slow.
Even when the web content is fully loaded (downloaded) the sidebar animation runs at single digit framerate on RTX2080Ti and 24thread CPU with 64GB of RAM.
When scrolling the content area itself the scrolling is so insanely laggy it just stops responding at some point completely, and the exact same thing happens when manually clicking the scrollbar.
Honestly, it feels almost like taunting to imply that no one has noticed this kind of performance during the plugin testing, or has not found it to be a problem. It’s not even that the scrolling is slow. It’s just that it stops working altogether.
Only benefit of the doubt I could give you is that the web UI framework you chose for the in-engine plugin doesn’t handle high poll rate input devices (My mouse has 1000Hz polling rate), which could possibly overload the scrolling with input requests and grind it to a halt. However this would still not explain why sidebar opens and closes at like 5FPS.
On the video you can clearly see it’s not the FAB content site itself, it’s the plugin and whatever framework it uses to (barely) draw the FAB site. Bridge plugin had the exact same issue, but at least the content was free. But there is no way I am spending money on service that has such low quality standards.