For caching to be effective, it needs to be more a long enough period of time to make it worth storing, but short enough that content is updated regularly.
A 30 second cache will increase overhead but for very little benefit. We have found that the 15 minute cache is the best balance between overhead and usefulness. We have "realtime" activity for cloud customers so you can always see who is active in a topic regardless of caching and when members log in, they are served live pages, not the cached ones.