Drawn as a Sprite
means drawing an Image
for every symbol, every digit, any art and the edge. So that's between 6 and 15 things being drawn for each card. Starling can do this: it can draw tens of thousands of items easily. But it all adds up. Managing draw calls in particular might be tricky depending how exactly the elements are drawn and how the cards are arranged.
Using a RenderTexture
requires GPU texture memory, but if you have the memory available it's a good trade off as it means far fewer things are being drawn. And as you are assembling them in-game you still get the benefit of not including card images in your app, so a smaller file size.