When opening, especially large assemblies with STEP, you can use filtering.
About different options related to imports, you can read in other entries. Use the search engine on the right.
Filter is used for this, you don't have to open the entire assembly at once, because it takes time. Once you turn it on, you can choose Generate 3D preview (1) and open only the specified file or several files. To open the file, select Import model (2).
You may not even generate a preview, only if you know the file name, select it from the tree on the left.
This really makes sense!