I think this has to do with the fact that proxy generation and transcoding tasks are “outsourced” to ffmpeg entirely. IIRC, you can even start working on your project while these tasks run in the background.
Rendering, however, even though it is “outsourced” to MELT is a different story because MELT is handling the effect application and the compositing frame by frame and then sends the frame to ffmpeg for encoding. So even though ffmpeg may use GPU MELT does not (yet?), hence the longer render time