Whenever I select a transcode in plex, for example, 4K to something like 480p, the speed in which it transcodes varies heavily. On my PC and Macbook, it's basically instantaneous, while on my iPhone it takes about 3-5 seconds for the transcode to finish, and on an android, even more time, up to 7 seconds.

I was told all transcoding is done on the server side, or whatever device is actually running the server. Not client side. So why is there a massive difference between transcoding video on a computer vs mobile? I'm using the same video and transcoding down to the same quality, Why is this?