Fast option uses the same windowed sinc smoothing filter (same as with the default flying edges surface extraction method). Faster option uses a different algorithm that performs smoothing during surface extraction, which is faster but less accurate. You can ask for more details in the topic where the feature was announced.
It might not be necessary to redo the complete segmentation. You can change the segmentation geometry and apply slight smoothing (e.g., using joint smoothing) and see if the result is satisfactory. It may not be optimal because some details may be lost that could have been present if the segmentation were performed at finer resolution from the beginning.