Response Time alerts have two critical states: Warning or Severe.
Once a test goes into Warning status, an alert is sent to the configured destination. If the performance degrades further until it exceeds the Severe threshold, the Severe alert will be triggered.
When test response times fall back below the configured thresholds, the Synthetic Classic Portal sends out an "Improved" notification. However, that notification is only sent once the performance is "good", meaning no longer exceeds the Warning threshold, to inform the recipients that the test is no longer in an alert state.
If performance improves so that it no longer exceeds the Severe threshold, but it still exceeds the Warning threshold, no Improved notification is sent because the test is still in an alert state.