Since the version 1.5, the page thumbnails (previews) are generated by the GN4 server.

When the same page preview is requested by multiple clients and needs to be updated, only the first call update the preview, the other calls are queued until the call that is generating the preview finishes.

Other calls wait up to 30 seconds to the preview to complete. Should the waiting be longer, the warning is written in the IIS event log.

Exception type: LockTimeoutException 

    Exception message: Exception of type 'TeraDP.GN4.Common.LockTimeoutException' was thrown.

   at TeraDP.GN4.Common.TimedLock.Lock(Object o, TimeSpan timeout)

   at TeraDP.GN4.EditorialServer.EditorialServer.GetPagePreview(Int32 pageId, String optionsName, Boolean force, String& etag)