Variation In Performance When Using GetDirectories?

I support an inherited asp.net web application that retrieves files from a shared network drive. One particular folder [we'll call it FOLDERA] I've chosen to troubleshoot against contains about 300mb of files and multiple subfolders. FOLDERA is considered large for this application. Starting recently, the users have been having slow performance when retrieving files from FOLDERA on a production network share. I narrowed the code down to the GetDirectories method within the asp.net application.

RootDir.GetDirectories("*", SearchOption.AllDirectories);

In production, the FOLDERA read takes about 8-10 seconds. Prior to the recent performance degradation it was about 1 second. In the test environment it takes 1-2 seconds with the same amount of data.