After some experimentation I've found that each widget rendered in this manner causes a noticeable hit to performance and so a widget with several children (a frame with 8 buttons, an image, and some labels in my case) together cause the slowdown.

Is it possible for me optimize this? Rendering without transparency to an intermediary pixmap and then drawing that with transparency to the QGLWidget doesn't seem to help, which is counterintuitive to me.