There are 3 states a service check can be in - up, down and slow. "Slow" is an extra state which helps for reporting and alerting when services are responding slower than you would expect.
Whether a service is considered "slow" depends on the "Slow Threshold" you configure when creating/editing the service check. This defaults to 500ms and means any response time greater than 500ms will change the status of the check to "slow".
This can be configured from the "Fine Tuning" section when adding or editing a check: