Generally I would advise that you load all data into QlikView if it all fits in memory. Without all data you will not be able to do business discovery on all your data, so in theory you might miss out on interesting facts and details by excluding data already during reload. Instead you can limit the data in your application presentation.
There is no way to apply a WHERE clause limiting the data load from file. That means that all data from the file is loaded to QlikView. You can add a WHERE clause in the LOAD statement to limit the number of rows that you want to keep in QlikView. The process time that you gain by that is the time and effort going in to putting the data in QlikView memory.
In the LOAD statement you can add a WHERE clause after the source definition. The WHERE clause is a set of logical comparison deciding if rows are kept or not. For example like this;