There are few issues that might cause Tabular to work slow in general or some of the actions to be slow.
1. The machine it is installed on have lower than the Recommended specs. You can check them here.
2. The Main workbook and returns are saved on a shared network drive and the connection to this drive is slow. Due to the architecture of the app there is an intensive data transfer between DB on network drive and local PC and if the connection is bad this can affect performance.
3. If the problem persist please contact the support team and it will be good to send the log file as well and our team will try to further help.
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