I believe that adding a variable to show or hide each object (or calculate on condition) can have a negative impact on performance on the server's side.
This can impact on the rendering performance on the client's side. If enough object conditions are calculated on each tab and the amount of data is big enough it can even reach the point where the server does not have enough time to calculate conditions before the client renders the content, making some objects to remain hidden or calculations to remain undone.
My recomendation is that you do not add any variables to show or hide each object but just a variable to know the tab that needs to be activated (to switch between tabs). I beleive this to be much better for performance.
Calculating what is needed only for each tab can impact badly on performance. QlikView uses cache on the server side to keep on RAM the calculated objects and selections so that it is much faster to go to previous selections or the selections that another user has made. This means that it's actually better for performance to have objects calculated beforehand. Calculations will be faster and consecuently rendering will start earlier.
Hope this helps.