The OPC Client that is connected to the OPC Server will tell it the rate that it wants updates sent to it. The industry standard is one second. Now, if the barcode scanner is sending the barcode to the OPC Server unsolicited, that means the server is not sending a request to scan to the scanner, Then you can probably set the update rate in the client to something faster because the server is just sending what it has in cache to the connected OPC Client.
What is the server and the client that is being used. That will allow us to refine our recommendations to you.