Hi Bernd,
I think that you have your terminologies confused here. Fileshare is a server based version of the file handler which runs as a separate process on a network server machine. Your program running on a client machine will redirect its file I-O across the network to the fileshare server where the I-O will actually be done.
This is turned on with the directive CALLFH"FHREDIR".
You are specifying CALLFH"EXTFH" which is the default to use the local file handler. The FILESHARE directive that you are using has nothing to do with the fileshare server technology to which Michael is referring but instead simply causes the file handler to treat the files as though AUTOMATIC locking was turned on.
So you are running the program locally and trying to share files on a network drive. The normal cause of performance problems in this environment is caused by the opportunistic locking done by Windows between a client OS and a Server OS.
There is an article from a 3rd party vendor here which although doesn't reference COBOL specifically does explain the problem. If you must share files on a network drive then you should turn off the local caching and oplocks.
In this situation it is often recommended that you do use the Fileshare technology mentioned above to avoid this problem altogether.
Thanks