This issue is most often caused by incorrect resource settings. Make sure that the game is enabled (Options/Settings.../Resources/Games), and that data paths are set up for it on the Paths tab. Any number of archives and/or folders can be added as paths, and folders are loaded recursively, so the simplest configuration consists of just the data path of the game (e.g. K:/SteamLibrary/steamapps/common/Starfield/Data). If the same resource can be found under multiple paths, then the one listed first has the highest precedence.
By default, Starfield models are automatically converted to use internal geometry data, which is currently required by most of the mesh editing functionality and spells in NifSkope. This conversion can be disabled in the general settings under NIF, and meshes can be converted to either format (internal or external) with spells.
Initializing the asset database can potentially take a long time if the number of loose resource files is unusually large. On Windows, performance is impacted much more by the number of resource folders than the number of files, so the problem is most often caused by having all Starfield geometry data (over 350,000 folders, each containing one .mesh file) added as loose resources. In this case, if the geometries are under the main game data path, it is recommended to add archives manually under Options/Settings.../Resources/Paths, and additional data folders only when they are really needed, instead of the simple configuration that recursively loads all data. Note that the 'Add Archive or File' button allows selecting multiple files at once, and that it is allowed to add sub-folders like K:/SteamLibrary/steamapps/common/Starfield/Data/geometries/MyMod as data paths.
Using too high quality settings for image based lighting can also result in long processing times when the environment map is first loaded.
Running NifSkope under Wayland on Linux currently requires setting the QT_QPA_PLATFORM environment variable to "xcb":
QT_QPA_PLATFORM=xcb ./NifSkope
Additionally, there may be issues with anti-aliasing (MSAA) settings above 4x. For this reason, MSAA is limited to a maximum of 4x on Linux by default, but this can be disabled in the render settings.