Fileshare improves performance when remote COBOL applications are concurrently accessing shared data files across a network but it cannot improve performance when the COBOL data resides on the same machine as the application. In this configuration, there is actually a performance overhead when using Fileshare instead of the standard Callable File Handler.
Although UNIX machines can be used as remote data servers, it is not uncommon for the application itself to run on the machine where the data resides. For example, multiple users may log in, across a network, to a machine where they run applications which share the same data. In such a configuration, you encounter the performance overhead of using Fileshare, making performance slower than when using the standard Callable File Handler.