This evening I continued on the gPoly tool. The focus was again on the display of the background images, since they are a very important feature when drawing ground features or when checking if they are in the right location. I made some good progress and for small to medium sized airports it is working fine now.
But when I try to load the high resolution background image I have of Schiphol things go less smooth, especially when you zoom in far and the highest resolution needs to be loaded. So there is still some work to be done when selecting the resolution that should be displayed, but progress is being made. I should note that the background image of Schiphol is a GeoTIFF of almost 4 GB in size, so it should be expected that things will never be super smooth when working with such a big file.