I would appreciate some diligence investigating the root cause of the problem. I don't think we should commit a behavior change when there isn't a degree of confidence that it'll solve the stated problem.
Based on the discussion, we (Kihwal, myself, Arpit) expect df to be a cheap call. Given that, there is not a degree of confidence that this change will speedup health checks. I'd like to hear at least a theory as to why df would be slow in a spurious way before we add complexity.
I'm not going to burn bridges over this patch if someone really wants to commit it, but as I said before, I would much rather see a patch to add new metrics or instrumentation to root cause the problem.