Hello,
ISMs will cull individual instances on the GPU. This happens before Nanite rasterizes the instances, so, it helps improve the performance of Nanite. This is where the performance increase on the GPU comes from when using Nanite ISMs, as compared to individual Nanite Static Meshes.
While you can cull whole ISM components on the CPU, the distance you would want to cull them at will likely correspond to the distance that you want to unload the related streaming cell at.
Keep in mind that this advice is general. To get the best performance, profile frequently on your target hardware, and guide your optimizations based on that.
Please let us know if this helps.