The performance in the Vizlib Custom Report should be very close to the performance of a single native table in Qlik Sense. The reason is that the Custom Report actually use the native objects and hypercubes of Qlik Sense. The objects are created when you select the dimensions and measures. If you feel there is a performance issue with Custom Report please check these two things:
1. Create a native Qlik table using the exact same dimensions and measures as you select in the Custom Report. Is there any difference in performance? (The result will be cached by Qlik engine so you have to restart between tests.)
2. In the extension properties find the setting for “Defer layout update” (under Settings/Settings). This feature is essential when working with larger data sets since it limits the amount of times the values need to be recalculated. Enabling "defer layout update" will cause the Custom Report to wait with updating the table until the "Update layout" button is clicked. Thus allowing the user to make several selections in quick succession without having to wait for the data set to refresh.
Of course if you still experience issues with performance that you think is extension related, send an email describing the issue to firstname.lastname@example.org. If possible, include a sample QVF app.
Always start the Custom Report with a smaller preset selection of dimensions and measures in the default state. This way the initial load will be shorter.
When creating the initial master item table use a very small selection. (Since that table typically has a lot of dimensions and measures it can be slow with larger data volumes.)
For additional general tips, see the Best practice and performance optimisation in Qlik Sense