As it is specific to only SYSADMIN user, please check if debugging or diagnostic are not enabled for this user. You can also trace the user's session and can see what exactly is happening by following the below notes.
How To Create a User Event Trace by Using Profile Option: Initialization SQL Statement - Custom (Doc ID 170223.1)
Oracle Application Server Diagnostic Tools and Log Files in Applications Release 12 (Doc ID 454178.1)
hope it helps.
May be there is lot of workflow mail notifications, you need to purge the notifications if not required
Is there any specific document there that says its related to workflow mail notifications and we need to purge the notifications. please help on this.