Проверить: Size limit on the user mailbox, either set at the database or mailbox level. Bandwidth limit between user and server.
Consider: Disabling cached mode, to have just a "1 way" data stream at the moment, eg) user workstation > Exchange, rather than user workstation > Exchange > user .ost file
Importing the pst using the New-MailboxImportRequest powershell command, rather than through Outlook.