Loading bigger Excel files can be painfully slow. Try the following: Fill 50 columns, 50000 rows in Excel with random numbers. That should give an excel file around 50MB big. Add this as a data table. It takes around 40s to even open the "Open" dialog window. Loading of the data then takes around 30s. Now save exactly that file from Excel as .txt and add the .txt as data table. the "Open" dialog appears instantly. Loading the data takes < 5s. It seems there's a lot of room for improvement in the load time.
Tested with Excel 2016, Spotfire 7.11. HF-004