This database mirroring event is issued on the principal server instance to indicate that the age of the oldest unsent transaction has reached a user-specified threshold value. Typically, this event occurs because the performance of the system has changed. Either the bandwidth between the two systems has decreased, or the load has increased.
The age of the oldest unsent transaction is a performance metric that can help you evaluate the potential for data loss as measured by the number of minutes of unsent transactions. This metric is especially relevant for high-performance mode sessions. However, this metric is also relevant for high-safety mode session when mirroring is paused or suspended because the partners become disconnected.