Slow I/O on one node during bulk indexing

During Bulk indexing, HQ is showing the following for one of the nodes in
my cluster (20nodes in my cluster). While every node has the value around
3000, this one node has value around 13000. Can some one Please explain
what this means and what should be the ideal value for this and there are
any settings to tune to control this?

High values indicate slow I/O.
Keys: stats.indices.flush.total_time_in_millis / stats.indices.flush.
total
Value: 65485927 / 5037 = 13000.97816160413
Thresholds:
13000.97816160413 <= 750 = Pass!
13000.97816160413 <= 1500 = Warning!

Thank you.

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