It may happen that a project cannot be opened until minutes of waiting have elapsed. In such cases it may help to use the function “File/ Clean up …” or to delete the contents of the /bin and /Libs folders. These files will be recreated the next time the project is opened.
But the problem could also be that you used the /Data Files folder, which is monitored by Katalon Studio itself, to store all your data needed for or created during your test execution. Please note that this may be an obvious idea, but not a very good one, as Katalon Studio searches this folder for usable data definitions when opening the project, which can be displayed in the Tests Explorer. This can take a lot of time (possibly several minutes to hours!). So it’s better to think of your own folder name for such purposes, which is not (yet) used by Katalon Studio, for example “Materials” (like kazurayam in his Materials library) or “Resources” or just “Results”.
Even an extensive content of the Reports folder can slow down the opening of the project. So if necessary, this folder should also be cleaned up occasionally.