There is aways a balance between available resources, network contention, data change rates, etc. For async, there is not as much pressure to have a "real-time" duplicate on the DR...if there was, you should just use sync SM, SMBC, or even MCC.
As I mentioned earlier, what you CAN do, may not always work depending on the environment. 5 minutes for individual FlexVol volumes is where we find the best compromise to supportability and meeting customer requirements.
With more "complex" volumes, such as FlexGroup volumes, we limit to 30 minutes because we want to be sure we can replicate the volume even when there are a high number of constituent volumes.
If a scheduled SnapMirror replication event attempts to start, while the previous one is still in process, you will receive an error message.