In retrospect I wouldn't have wrapped ControlsFX. There are major implementation mistakes inside the library. For example, it can download FontAwesome on the main thread in some situations. Things that never happen in B4X code.
I wasn't aware to these issues when I wrapped it.
This might be the reason for the slowness you see. My recommendation is to remove the library and implement a similar control yourself. It shouldn't be too difficult to create one based on two CustomListViews.