How to tell if QTableView cell is visible ?

I have a big QTableView that has scroll bars. Many rows and columns are not visible, meaning you have to scroll to seem them.

Does anybody know how to tell if a cell at a particular row,column must be scrolled to in order to be seen ?

Here is some background on what the project:
The App is being used in a production system but takes up lots of CPU. The App is basically a table which is a QTableView. The users have typically a few hundred rows. But only about 30 or so rows are visible at a time due to screen size of the monitor. The rest of the rows can be scrolled to.

We have real time data coming in and each time data comes in cells are updated with that new data. What happens now is the data contains a key which allows me to located the cell of the data. Then the cell is updated with the data itself or some calculation on the data ( the calculation itself can take a fair bit of time ). If a cell is not visible we want to perform no calculations at all and not set the cell at all. The data comes in fast enough and refreshes enough from the real time system that if the user scrolls to cells not visible they will fill with the updated data as very shortly new data will come in. Discarding the updates from the real time system therefore is not a problem.

Does this make any sense ? If I am not being clear let me know.