Hello Jor, thank you for your comment. The "Wait background load" message appears during the time the panel part waits for answer from the background part of the add-on, at Firefow start up.
This is usually a symptom of Firefox taking time to load and to fully start, or slowly answering the BSP2 background task request to get the last version of bookmarks if you enabled the option to retrieve it on each restart.
This can often happen if you have many windows and many tabs that Firefox reopens at each restart.

If you want to get more details, there are some traces on start that we can activate, see BSP2 wiki https://github.com/aaFn/Bookmark-search-plus-2/wiki. You can also open an issue there if needed that we investigate.