If you perform calculations in the user interface that is performed on a row-level, like Price*Qty, it's better to move this to the reload script. This will reduce the resources needed to calculate the object.
Creating flags for things of intrest will also be of help. If the flags are numeric instead of text will make things even better since comparing text is slower than numerics. If the calculations are used in the dimensions-part of an object makes this even more valuable.
This is especially important if the volumes of users and/or data is growing. In most cases QlikView will perform magic and but when it's time to scale it's a good idea to do this before it becomes an issue.
Cheers from the Scalability Team