You receive this message when the storage device performance improves. This event is generated as the device latency decreases. This occurs when either the latency ratio to the last time the log was updated is 5 or it doubles.
For more/related information, see:
Impact
The storage device performance improves.
Example
The following is an example of the message received when this occurs:
Device naa.5000c5000b36354b performance has improved. I/O latency reduced from 19403 microseconds to 1201 microseconds.