Besides of having Session Recovery enabled you should enable Calculation Condition in your large tables. Imagine a user Qliks on Clear All with that table open. Since it can be millions of records on it, it'll froze the machine.
In charts, in the General tab, you can find the Calculation Condition. Set it up like count(Distinct Business Unit)<=1 or
count( distinct RecordNumber)<=1000. By doing that you'll save frozen times developing and using it.
You can set up error messages too, like: Please select a Business Unit only.