Are you working while connected to a published TDE
If not are you working from an extract of data
Another thing to try is save shutdown and restart computer.
I have found that using Tableau on a 4GB Ram laptop while using other programs like outlook internet and having multiple windows of tableau open does really drain the memory and results in out of memory errors
Tableau can display a half million records quickly.
SLow performance can be caused by things you are doing in the sheet. Blends, table calcs, etc.
A SEARCH of this forum or of Tableau knowledge base articles will turn up a lot of discussions about ways to speed up your performance.