I would also suggest to check the log files. These log files are found in Documents --> My Tableau Repository -->Logs.
Rename Logs--> Logs_Backup
Now open your slow desktop/ worksheet. Check the Logs folder that Tableau created. You might be in touch with Tableau support but I am sure they shall need your log files.
Have you tried copying your repository to your desktop, then pointing your repository within Tableau to the Desktop version of it?