A little known fact that can cause Windows client performance issues: Outlook PST files are not supported on network drives and were devised back in the days of Exchange 4.0 and were intended to be used for local storage only.
Although it is possible to store PST files on a network drive or a UNC path this is not a supported configuration.
“A .pst file is a file-access-driven method of message storage. File-access-driven means that the computer uses special file access commands that the operating system provides to read and write data to the file.
This is not efficient on WAN or LAN links because WAN/LAN links use network-access-driven methods, commands the operating system provides to send data to or receive from another networked computer. If there is a remote .pst (over a network link), Microsoft Outlook tries to use the file commands to read from the file or write to the file, but the operating system then has to send those commands over the network because the file is not on the local computer. This creates a great deal of overhead and increases the time it takes to read and write to the file. Additionally, the use a .pst file over a network connection may result in a corrupted .pst file if the connection degrades or fails.”