Rendering the weather column when WorstChildHealthMetric is used causes the last six builds of any contained jobs (even in subfolders) to be loaded into memory if they were not already. This can cause slow rendering times for the home page in a large installation.
Perhaps WorstChildHealthMetric.DescriptorImpl should not override createDefault. That would at least ensure that by default folders would not do expensive computations on child jobs. This would be helpful in installations contains lots of jobs grouped into folders (and no top-level jobs).
Of course administrators can manually remove all health metrics from folders, but they have to know to do this, when it is by no means obvious that this is the cause of slow view rendering. I think you should need to opt in to something which may add performance overhead.
Would do no good on folders whose configuration has already been saved at least once, and healthMetrics persisted. But would be useful on newly created folders.
Also help.html of any metric which loads builds should mention this fact.