My guess is that it is Sense protecting you from sending a download to the client that will slow things down horrendously, and also cause it a lot of time to calculate. When the table is rendered on the screen only the values you can see are calculated, and cache is created to allow smooth scrolling. If you export then every single value needs to be calculated.
If you need to export every row then you are better off doing this with a STORE statement in your load script. This can only do one table at a time, so it may be that you need to do some ApplyMap or JOIN statements in the load to get the data in the right state.
Hope that helps.