If your current scene contains game objects making use of the LOD Group component, you have two different options for assigning these objects:
1) Assign the root game objects that have the LOD Group component (recommended). In this scenario, your Streamable Grid should generally only have a single LOD Group, and you will assign these game objects to this LOD Group.
2) Assign each LOD game object to a separate LOD Group on the same World Grouping. Usually the Regex Pattern Matching technique is best for this option. You will just need to run separate Assignment Operations for each LOD, ensuring that the pattern is set to only match the LOD you want.
Keep in mind that while option 2 is more memory efficient, option 1 is usually better during the development process. This is because the World Designer Tool does not currently have an easy way to perform cross LOD edits of game objects.
For instance, if you want to reposition a game object belonging to a Streamable Grid with multiple LOD Groups, currently you need to load the Asset Chunks that game object is part of for each LOD Group in turn, modifying each LOD game object's position in the same way, to ensure all LODs stay in sync.
If using option 2, also keep in mind that you can only place different objects on the same Grouping if they have the same number of LODs, as each Grouping uses the same number of LODs for all Asset Chunks. If you break this rule, SAM will attempt to load LOD assets that do not exist, which will result in exceptions!
The performance of the Assignment Operation may seem slow at times, however please bear in mind that this operation (and the other Advanced Operations) are performing batch Asset Database modifications, which Unity is not always setup properly for.
Several optimizations have already been made to make these operations as fast as possible, however there is only so much that can be done. If speed is a concern, consider using Addressable Assets, which should generally be faster then using non Addressable Scenes or Prefabs. Using Non Addressable Scenes will typically be the slowest option in terms of the World Designer Tool. You can somewhat overcome this by creating Prefab Instances within your scenes, which the World Designer Tool can work on directly in many cases rather than the Scene Assets.
If you elect to use the Addressable System to configure and load your Asset Chunks, please remember that depending on which Addressable Chunk Streamer you are using, you will need to generate the appropriate Asset Manager before using the World Designer Tool.
You can do this via the Asset Menu Command using Assets - > Deep Space Labs -> SAM -> Regenerate Default Addressable Prefab Asset Manager or Assets - > Deep Space Labs -> SAM -> Regenerate Default Addressable Scene Asset Manager
You can read more about using the Addressable System with SAM via its dedicated chapter.
After the conversion process, your new Asset Chunks should be ready to load, however if you have created 1 or more new Streamable Grids/World Groupings during this process, it's important to remember that you will also need to add corresponding Groupings to your Active Grid Component (assuming you are using one).
You should have an understanding of how to do this already if you went through the Getting Started Tutorial. Remember that in order to control the range of Streamable Cells/Asset Chunks that are loaded for each Grouping, you will likely need to create new Loading Blueprints (in your existing Loading Blueprint Repository), and then assign these to the new Groupings on your Active Grid.
It is very common to have a different Loading Blueprint for each Active Grid Grouping, so feel free to make as many Blueprints as needed and to experiment with different setups.