When you add any condition in the where statement of qvd it is not optimized anymore so reload is slow. The only solution is to load qvd without conditions (optimized). also can you please kindly write your example that you are writing in your code it help us to fix up this issue.
Better way is first load the QVD as normal from the data base like reading Raw Data and make QVD. After this do the transformation and other filter selection in the QVD that will reduce the load time also it is more convenient to you to use this QVD. And if the qvd reading raw data from the source data it will take time depends on your data base rows.
Also, you can check. If you do not need all the data. Then read only required rows. Like say if you have transaction data for various Years, you need only current and previous years data. Then you must limit yourself to read only the current and previous years data. This will bring down the load times. However it depends on your context as well.
