Luanti has a cogwheels button on the top-right side of the UI. Click that, go to Graphics and change the Maximum FPS to what you want and click the Set button to save the change. At the same time, make sure VSync is disabled, to see if Luanti can handle the maximum FPS you set.
That said, there's an open issue about Luanti having FPS issues since version 5.9.0. The issue is still open, with 5.10 being the latest Luanti release, and that's all we have at the moment.