ELI5: Why is it quicker to download a movie than to transcode it?

I often download movies which won't play correctly on certain devices and need to transcode them to allow playback. I find it's actually quicker to download again a different format rather than transcode the original, even for comparable file sizes.

Download of a 200mb file = 5-10 minutes. Transcode that file = 30 minutes or more.