This is a performance-monitoring event.
Slow Content Download Events identify slow downloads that are related to download performance that can directly influence Web server performance and download times.
Slow Content Downloads, if not pinpointed, can bring the server down by causing excess download activity causing slower response times.
Slow Content Download Events provide a different perspective on problems (view outside problems through the eyes of PHP). Despite the fact that the code may be running okay, this Event indicates what other outside problems (i.e. network, database, web services, file system etc.) you may have, based on the content download’s behavior.
Generates "Slow Content Download" events whenever an invocation of the API function fpassthru() rises above the given threshold. The default values are, 5000 msec for moderate, 10000 msec for severe alerts.
The Zend Download Server (ZDS) is the optimal solution for handling large downloads. Use the ZDS API to optimize download performance and reduce download traffic from your Web application.