I don't know why it is, but SMB on OS X Server is slower and less reliable than the AFP that it replaces. Despite Apple making it the default for OS X to OS X Server file sharing connections, AFP...
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Sometimes an OS X Server will have very poor SMB file sharing performance - whilst I haven't been able to ascertain with 100% certainty what causes it, something that may be a factor is the use of SMB 3 connections...
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